Fiber optic interferometric sensor array

ABSTRACT

The invention relates to an apparatus for multiplexing Fiber Optic Interferometric Sensors (FOISs), which apparatus comprises means (1) forming the an optical source for providing m*n distinct wavelengths λ 11 , λ 12 , . . . , λ 1n , λ 21 , λ 22 , . . . , λ 2n , . . . , λ m1 , λ m2 , . . . , λ mn , the optical source including means for generating m groups ( 111, 112, . . . , 11   m ) of light pulses, each group  11   j  (1≦j≦m) being made up of n light pulses P k (λ j1 ), P k (λ j2 ), . . . , P k (λ jn ) of distinct wavelengths λ j1 , λ j2 , . . . , λ jn , the n light pulses P k (λ j1 ), P k (λ j2 ), . . . , P k(λ   jn ) being time delayed one from another, Fiber Optic Interferometric Sensors distributed in m groups ( 51, 52, . . . , 5   m ) of n sensors each, then sensors of a same group being located at the same point and set into a star like arrangement around the input point of the group, and each of the n sensors of a same group  5   j , 1≦j≦m, being associated with wavelength selective means so that it is interrogated by one light pulse only among the pulses P k (λ j1 ), P k (λ j2 ), . . . , P k (λ jn ) of distinct wavelengths respectively λ j1 , λ j2 , . . . , λ jn .

The invention is related to a Fiber Optic Interferometric Sensor (FOIS)array implementing wavelength and time division multiplexing.

The invention can be dedicated to seismic sensing applications where thesensor arrays are made of several receiver points located at differentplaces; each receiver point may contain several sensors. Neverthelessthe invention is not limited to seismic sensing applications and can bededicated to any sensing applications involving an array of FOISs spreadinto groups of sensors.

Some external perturbations applied on an optical fiber length (pressurevariations, temperature variations, vibrations, . . . ) cause the fiberlength and index of refraction to change, leading to a change of thetime propagation (or phase) of the light field propagating in the fiber.An FOIS is commonly made of two fibers : at its entrance, the light isdivided into two optical fields:

-   -   the optical field propagating in a fiber length submitted to the        perturbation to measure (an acoustic signal for example) : this        fiber length forms the sensing part of the FOIS,    -   the optical field propagating in a fiber length isolated from        the perturbation: this fiber forms the reference fiber of the        FOIS.        When the FOIS is submitted to an external perturbation, the        difference in time propagation, through the FOIS, of the two        optical fields will change. This change in time propagation (or        phase) difference is representative of the external perturbation        applied on the sensing part of the FOIS.

An FOIS interrogating system ensures the dynamic reading of the phasedifference between two optical fields. It is made of an optical sourceproviding the light, a telemetry fiber bringing the light to the FOIS, areturn fiber bringing the light from the FOIS to the photo-detector onwhich the combination of the two optical fields leads to aninterferometric signal bearing the information on the phase differencebetween the two optical fields, and an electronic demodulation systemcomputing the phase difference from the photo-detector outputinterferometric signal. In order to increase the FOIS sensitivity to themeasurand, that is to say the change in phase, involved by themeasurand, of the optical field propagating in the sensing arm of theFOIS, the sensing fiber length can be packaged on an external bodybringing measurand amplification. In the next sections the sensing partof the FOIS will be denominated the ‘sensor’.

In some applications, the sensors involved in an array can be as many asa few thousands. The decision to go for such arrays mainly depends ontheir cost and reliability.

In a FOIS array, the interrogating system is ideally shared betweenseveral FOIS so as to reduce the array cost: the optical source, thetelemetry and return fibers, the photo-detector, and the electronicdemodulation system can interrogate many FOISs. A FOIS array is thusmade of one or more optical sources, of which light is guided to a groupof FOISs. Each FOIS locally brings its information on the optical fieldthat goes through it. The return fiber brings the modified opticalfields to one or more photo-detectors. The interferometric signals fromthe different FOISs then need to be separated or demultiplexed.Demultiplexing of the interferometric signals is achieved by separatingthe optical fields associated with each FOIS:

-   -   The optical fields can be separated in the time domain if each        optical field associated with an FOIS occupies a different time        window: Time Division Multiplexing (TDM) is thus achieved.    -   The optical fields can be separated in the wavelength domain if        each optical field associated with an FOIS has a different        wavelength: Wavelength Division Multiplexing (WDM) is thus        achieved.    -   A combination of TDM and WDM techniques can be implemented.

Several systems were demonstrated to arrange FOIS in arrays. Among them,an implementation of the Time Division Multiplexing (TDM) technique iswidely employed. It is based on a scheme illustrated by FIG. 1 and usinga reference interferometer associated with the fiber optic sensinginterferometer (the FOIS). Referring to FIG. 1, the system 10 is made ofan optical source 100 of which light is coupled in an optical fiber 170,an optical switch 120 generating a light pulse P from the continuouswave light emitted by the optical source 100, a reference interferometer130 isolated from the signal to measure, a fiber optic sensinginterferometer 140 forming the FOIS, and a photo-detector 70. The lightpulse P is lead to the reference interferometer 130 by the optical fiber170. The reference interferometer 130 is made of two fiber arms formingtwo optical paths 130 _(a) and 130 _(b) of difference D. The opticalpath 130 _(a) comprises means 13 ₁ capable of phase modulating theoptical field that goes through it so as to generate a carrier frequencyfor the signal to measure. Several techniques are employed in the stateof the art, among which one which integrates an Acoustic Optic Modulator(AOM) as means 13 ₁: the AOM shifts the optical carrier frequency of alight propagating through it by a frequency fc (characteristic of theAOM), fc˜30 MHz. At the output of the reference interferometer 130, thepulse P gives rise to two optical pulses P_(a) and P_(b), delayed onefrom another by a time t(D) corresponding to the time propagation oflight through the optical path difference D. Pa and Pb pulses are thenguided to the fiber optic sensing interferometer 140. The FOIS 140 ismade of two fiber arms forming two optical paths 140 _(a) and 140 _(b)of difference D′ very closed to D. The optical path 140 _(b) issubmitted to an external perturbation M that modifies it, and thusmodifies the phase of an optical field that goes through it. The opticalpath 140 _(a) is isolated from the external perturbation M. At the exitof the FOIS 140, two pulse couples are rose up: [P_(a,a), P_(b,a)] and[P_(a,b) et P_(b,b)]. The pulses P_(a,a) and P_(b,a) respectively resultfrom the pulses P_(a) and P_(b) that traveled through the short opticalpath 140 _(a) of the FOIS 140. The pulses P_(a,b) and P_(b,b)respectively result from the pulses P_(a) and P_(b) that traveledthrough the long optical path 140 _(b) that sees the influence of theexternal perturbation M. Thus the two pulse couples [P_(a,a), P_(b,a)]and [P_(a,b) et P_(b,b)] are delayed one from another by a time t(D′)corresponding to the time propagation of light through the optical pathdifference D′; and the two pulses of each couple are delayed one fromanother by a time t(D) corresponding to the time propagation of lightthrough the optical path difference D. The times t(D) and t(D′) beingvery closed, the two light pulses P_(a,b) (that saw the externalperturbation influence M) and P_(b,a) (isolated from the externalperturbation M) are synchronized and the two optical fields caninterfere on the photo-detector 70. Their interfering signal givesinformation on their phase difference, and consequently on the externalperturbation M.

It can be noticed that the reference interferometer 130 can be eitherplaced before or after the sensing interferometer 140.

As an example, the document <<Fiber interferometric sensor arrays withfreedom from source phase induced noise>> in <<Optics Letters>>, vol.11, July 1986, n^(o) 7, pp. 473-475 (denominated Document D1) presents aTime Division Multiplexing Technique based on the measurement principledescribed above. FIG. 2 a illustrates the implemented TDM technique; itincludes an optical source 100, an optical switch 120 generating a lightpulse P in a telemetry fiber 170 a , a reference interferometer 130comprising means 13 ₁ capable of phase modulating the optical field thatgoes through it, and a photo-detector 70. The light pulse P is lead bythe telemetry fiber 170 a to a group of n FOISs (Mach Zenderinterferometers) 141, 142, . . . , 14n. The reference interferometer 130is here placed after the FOISs. Each of the FOIS 141, 142, . . . , 14nrespectively comprises a short optical path 141 a, 142 a, . . . , 14nand a long optical path 141 b, 142 b, . . . , 14n forming the sensingpart of the interferometer. The optical path difference between theshort and long paths (141 a, 141 b), (142 a, 142 b), . . . , (14na,14nb) of each FOIS 141, 142, . . . , 14n is equal to D′ and is veryclosed to the optical path difference D between the short and long paths130 a and 130 b of the reference interferometer 130. The interrogationof each FOIS 141, 142, . . . , 14n is then based on the principle ofinterrogation of one FOIS as described above by FIG. 1. Indeed the lightpulse P at the entrance of the fiber optic coupler 161 a gives rise atits exit to two pulses P₁ and P′₁, P₁ being lead to the interferometricsensor 141, and P′₁ being lead to the fiber optic coupler 162 a. Thepulse P′₁ gives rise to two pulses, P₂ and P′₂, at the exit of the fiberoptic coupler 162 a, P₂ being lead to the interferometric sensor 142,and P′₂ being lead to the fiber optic coupler 163 a. And so on, thepulse P′_(n−2) at the entrance of the fiber optic coupler 16(n−1)a givesrise at its output to two pulses P′_(n−1)=P_(n) and P_(n−1), P_(n−1)being lead to the sensing interferometer 14(n−1), and P′_(n−1)=P_(n)being lead to the sensing interferometer 14n. The n pulses P₁, P₂, . . ., P_(n) respectively formed at the entrance of the n FOISs 141, 142, . .. , 14n can thus interrogate respectively the n FOISs 141, 142, . . . ,14n the same way as described above in the method illustrated by FIG. 1for one pulse interrogating one FOIS. The pulses coming out from eachFOIS 141, 142, . . . , 14n are then coupled in the return fiber 170 b,respectively by the fiber optic couplers 161 b, 162 b, . . . , 16(n−1)b,and are then lead to the reference interferometer 130. At the exit ofeach n couple (FOIS 141, reference interferometer 130), (FOIS 142,reference interferometer 130), . . . , (FOIS 14n, referenceinterferometer 130), the n pulses P₁, P₂, P₃, . . . , P_(n) give rise ton interferometric signals on the photo-detector. In addition, the (n−1)fiber sections 171, 172, . . . , 17(n−1) added in between each fiberoptic coupler 161 a, 162 a, . . . and 16(n−1)a, that is to say inbetween each FOIS 141, 142, . . . , 14n ensure that the pulses P₁, P₂,P₃, . . . , P_(n) are time delayed one from another at the entrance ofthe FOISs. The n interferometric signals resulting from the n FOISs 141,142, . . . , 14n are thus time delayed one from another on thephoto-detector 70 and can be time gated. The (n−1) fiber optic sections171, 172, . . . , 17n must be of accurate length so that the n pulsesbearing the interferometric signals from the n FOISs don't overlap onthe photo-detector 70. More precisely, the optical paths formed by thefiber optic sections 171, 172, . . . , 17(n−1) added in between theFOISs 141, 142, . . . , 14n are here made equal to D″, D″ being veryclosed to the optical path difference D in the reference interferometer130, and consequently very closed to the optical path difference D′ ineach FOIS 141, 142, . . . , 14n. The n pulses carrying theinterferometric signals from the n FOISs are thus time delayed by t(D″)corresponding to the time propagation by the light through the opticalpath D″ (t(D″)˜t(D)˜t(D′)).

It is obvious that the TDM FOIS interrogating system illustrated by FIG.2 a requires to add some fiber optic sections of accurate lengths, inbetween each FOIS.

FIG. 2 b shows another implementation of this TDM technique, in whichthe Mach Zender FOISs 141, 142, . . . , 14n of FIG. 2 a are replaced byMichelson FOISs. The light pulse P, at the output of the optical switch120 is lead by the optical fiber 170 to a circulator 160, and then leadby the telemetry fiber 170 a to an array of n Michelson FOISs 141, 142,. . . , 14n. In accordance with FIG. 2 a, n pulses P₁, P₂, P₃, . . . ,P_(n) time delayed one from another, are respectively formed at theentrance of the n Michelson FOISs 141, 142, . . . , 14n. Each FOIS 141,142, . . . , 14n comprises a short optical path, respectively 141 a, 142a, . . . , 14na terminated by a mirror 54, and a long optical path,respectively 141 b, 142 b, . . . , 14nb (forming the sensing part of theFOIS) terminated by a mirror 54. The round trip difference between theshort and long optical paths (141 a, 141 b), (142 a, 142 b), . . . ,(14na, 14nb) of FOIS 141, 142, . . . , 14n is equal to D′ and is veryclosed to the optical path difference D in the reference interferometer130. The light pulses P₁, P₂, P₃, . . . , P_(n) respectively formed atthe entrance of the n FOISs 141, 142, . . . , 14n respectivelyinterrogate the FOIS 141, 142, . . . , 14n the same way as describedabove. After reflection on the mirrors 54, the light pulses coming outfrom each FOIS 141, 142, . . . , 14nare coupled back in the opticalfiber 170 a by the fiber optic couplers respectively 161 a, 162 a,16(n−1)a, and lead by the circulator 160 to the return fiber 170 b, andthe reference interferometer 130. At the exit of each n couple (FOIS141, reference interferometer 130), (FOIS 142, reference interferometer130), . . . , (FOIS 14n, reference interferometer 130), the n pulses P₁,P₂, P₃, . . . , P_(n) give rise to n interferometric time delayedsignals on the photo-detector 70. Each pulse carrying theinterferometric signal of each FOIS can thus be separately time gated.As the TDM FOIS interrogating system illustrated by FIG. 2 a, the systemillustrated by FIG. 2 b, requires to add (n−1) fiber optic sections 171,172, . . . , 17n−1 of accurate lengths, in between each FOIS 141, 142, .. . , 14n. The (n−1) fiber optic sections each forms an optical pathD″/2 (that is to say a D″ round trip optical path).

FIG. 2 c shows a third implementation of this TDM technique, in whichthe FOISs 141, 142, . . . , 14n are Michelson interferometers, and inwhich the (n−1) fiber optic sections 171, 172, . . . , 17(n−1) of FIG. 2a or 2 b ensuring the time delays between the interference pulses onphoto-detector 70 are suppressed. The time delays between the ninterference pulses from FOIS 141, 142, . . . , 14nare ensuredrespectively by the fiber optic sections 141 b, 142 b, . . . , 14(n−1)bforming the sensing long arm of the FOIS 141, 142, . . . , 14(n−1). FOIS141, 142, . . . , 14(n−1) comprises a short optical path, respectively141 a, 142 a, . . . , 14na terminated by a mirror 54 and a long opticalpath, respectively (141 b+142 a), (142 b+143 a), . . . ,(14(n−1)b+14na), 14nb terminated by a mirror 54. The round tripdifference between the short and long optical paths (141 a, 141 b+142a), (142 a, 142 b+143 a), . . . , (14(n−1)a, 14(n−1)b+14na), (14na,14nb) in the FOIS 141, 142, . . . , 14n is equal to D′, and is veryclosed to the optical path difference D in the reference interferometer130. In this specific way of implementation no fiber optic sectionsneeds to be added in between each FOIS, but the X-talk phenomenonbetween the sensors becomes significant since the pulse P_(i+1) enteringFOIS 14(i+1), (1≦i≦n−1) is guided by the fiber section 14ib forming thesensing part of FOIS 14i. The interferometric signal of FOIS 14(i+1) isthus marked, to some extent, by the FOIS 14i response. In addition tothis, if the fiber optic section 14ib forming the sensing part of FOIS14i is cut (which is likely to happen since this section can be highlystressed), one losses all the FOISs following FOIS 14i on the telemetryfiber.

To sum up, the techniques implementing Time Division Multiplexing, havethe drawbacks that they:

-   -   Either require to add a number of fiber sections of accurate        lengths in between each FOIS (FIGS. 2 a et 2 b); the number of        added fiber sections is equal to the number of multiplexed        FOISs.    -   Either lead to arrays with poor X-talk performances between the        multiplexed sensors, and reduced reliability (FIG. 2 c).

The Time Division Multiplexing technique presented by document Dl can becombined with a Wavelength Division Multiplexing technique: the document<<Remotely pumped and interrogated 96 channel fiber optic hydrophonearray>>, published in <<Optical Fiber Sensor Conference 16>>, November2003, pp 760-763 (referred below as document D2) describes animplementation of combined TDM and WDM techniques. Referring to FIG. 3,the lights emitted by six optical sources 101, 102, . . . , 106 ofdifferent wavelengths respectively λ1, λ2, . . . , λ6 are multiplexed ona telemetry fiber 170 by a wavelength multiplexer 107 a. An opticalswitch 120 periodically generates a light pulse containing the lights ofdifferent wavelengths λ1, λ2, . . . , λ6. The light pulse is lead to areference interferometer 130 (identical to the one described in FIGS. 1and 2). The light of wavelength λ1 (respectively λ2, . . . , λ6) isextracted from the telemetry fiber 170 by the optical component 121 a(respectively 122 a, . . . , 126 a); the pulse P(λ₁) (respectivelyP(λ₂), . . . , P(λ₆)) is then lead to the sub-array 121 (respectively122, . . . , 126) containing 16 FOISs. In each sub-array 121, 122, . . ., 126, the 16 FOISs are TDM multiplexed as described by document D1. Atthe output of sub-array 121 (respectively 122, . . . , 126), the lightof wavelength λ1 (respectively λ2, . . . , λ6) that carries the 121sub-array FOIS responses (respectively 122, . . . , 126) is inserted onthe return fiber 171 by the component 121 b (respectively 122 b, . . . ,126 b). The return fiber is terminated by a wavelength demultiplexer 107b that separates the lights of different wavelengths λ1, λ2, . . . , λ6.Each wavelength λ1, λ2, . . . , λ6 is lead to its photo-detector,respectively 71, 72, . . . , 76. On photo-detector 71 (respectively 72,. . . , 76) the 16 pulses carrying the interferometric signals from the16 FOISs of sub-array 121 (respectively 122, . . . , 126) are timedelayed one from another and can be time gated for demultiplexing. Thecombination of TDM and WDM techniques makes for higher multiplexingdensity. Nevertheless, as pointed out in Document D1, the TDM techniqueimplemented in Document D2 (identical to that implemented in DocumentD1), has the drawbacks that, in each sub-array 121, 122, . . . , 126:

-   -   It either requires to add a number of fiber sections of accurate        lengths in between each FOIS (FIGS. 2 a et 2 b); the number of        added fiber sections is equal to the number of multiplexed        FOISs.    -   It either leads to arrays with poor X-talk performances between        the multiplexed sensors, and reduced reliability (FIG. 2 c).

Seismic applications require that the sensors are located at specificplaces in the array. Also ease of fabrication of the array, reliabilityand low X-talk between sensors are great benefits.

For seismic applications such as <<Deep Ocean Bottom Cable>> (DOBC), thearrays are made of several Receiver Points (RP), called nodes, distinctone from another and geographically spaced by a given distance. Eachnode comprises four sensors geographically located at the same point;the four sensors, one hydrophone and three geophones, form a ReceiverPoint or measurement node. The X-talk level required between the foursensors is very low (<−50 dB).

For such applications, the systems proposed by Documents D1, D2 and D3,and more generally in the state of the art, show some drawbacks amongwhich the fabrication constraints (need to add some fiber sections ofaccurate length in between each sensor, some sensors being located atthe same point), poor X-talk performances and reduced reliability.

The present invention is aimed at getting rid of the drawbacks,mentioned above, involved when multiplexing FOISS. More specifically,its objectives are:

-   -   To get rid of the fabrication constraints to add fiber sections        of accurate length in between each Fiber Optic Interferometric        Sensor (FOIS) to interrogate (whether the sensors are located at        the same node or not).    -   To keep X-talk and reliability performances.

The invention concerns an apparatus for multiplexing Fiber OpticInterferometric Sensors (FOISs), which apparatus comprises:

-   -   means forming the optical source for providing m*n distinct        wavelengths λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n),        . . . , λ_(m1), λ_(m2), . . . , λ_(mn), which means comprise        other means for generating m groups of light pulses, each group        11j (1≦j≦m) being made up of n light pulses P_(k)(λ_(j1)),        P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) of distinct wavelengths        λ_(j1), λ_(j2), . . . , λ_(jn), the n light pulses        P_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) being time        delayed one from another,    -   Fiber Optic Interferometric Sensors distributed in m groups of n        sensors each, the n sensors of a same group being located at the        same point and set into a star like arrangement around the input        point of the group, and each of the n sensors of a same group        5j, 1≦j≦m, being associated with wavelength selective means so        that it is interrogated by one light pulse only among the pulses        P_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) of distinct        wavelengths respectively λ_(j1), λ_(j2), . . . , λ_(jn).

Other characteristics, objectives and advantages of the presentinvention will become apparent in the following detailed descriptionillustrated by the appended figures, given as non restrictive examplesand among which:

FIG. 1 shows a scheme of interrogation of one FOIS by a pulsed light

FIG. 2 a, 2 b, 2 c show different schemes of the implementation of theTime Division Multiplexing of several FOISs interrogated as described byFIG. 1 and in the state of the art;

FIG. 3 shows a scheme combining the TDM technique described by FIG. 2and a WDM technique for multiplexing a number of FOISs as usually donein the state of the art;

FIG. 4 shows a scheme of a system in accordance with the presentinvention, based on a combination of the TDM and WDM techniques, andcapable of interrogating one group (or node) of n FOISs localized at thesame point;

FIG. 5 is a representation of the light sent in the telemetry fiber thatis required to interrogate the aforementioned group (or node) of n FOISslocalized at the same point; this light is made up of n light pulses,each of which having a specific wavelength, and each of which beingdelayed one from another, the group of n pulses being periodicallygenerated at a repetition rate Tr;

FIG. 5 a, 5 b, 5 c, 5 d show schemes of some possible ways ofconstruction for means 1 capable of generating the n pulse-grouprepresented in FIG. 5

FIG. 6 is a representation of the group of n couples of light pulsesresulting from the group of n light pulses represented in FIG. 5, at theoutput of the reference interferometer;

FIG. 7 is a representation of the group of n*2 couples of light pulsesresulting from the n couples of light pulses represented in FIG. 6, atthe output of a n FOIS-node;

FIGS. 8 a and 8 b show schemes of possible ways of construction for onen FOIS-node of FIG. 4; the n FOISs of the node being spread out aroundthe input point of the node in a ‘star’ arrangement;

FIG. 9 shows a scheme of a system in accordance with the presentinvention, based on a combination of the TDM and WDM techniques, andcapable of interrogating m groups (or nodes), each made of n FOISslocalized at the same point, the distance or fiber length between anytwo nodes being allowed to be of any value;

FIG. 10 is a representation of the light sent in the telemetry fiberthat is required to interrogate the m*n FOISs; this light is made up ofm groups of light pulses; each group consisting of n light pulses ofdistinct wavelengths and time delayed one from another; the m groupsbeing generated at a repetition rate Tr;

FIG. 10 a, 10 b, 10 c, 10 d show schemes of possible ways ofconstruction for means 1 capable of generating the m groups of lightpulses described in FIG. 10;

FIG. 11 is a representation of the j^(th) group, 1≦j≦m, of n couples oflight pulses, resulting from the j^(th) group of n light pulsesrepresented in FIG. 10, at the output of the reference interferometer;

FIG. 12 is a representation of the j^(th) group, 1≦j≦m, of n*2 couplesof light pulses, resulting from the j^(th) group of n couples of lightpulses represented in FIG. 11, at the output of the j^(th) n FOIS-node;

FIG. 13 shows a scheme of one possible way of construction for one nFOIS-node j, 1≦j≦m, of FIG. 9; the n FOISs of the aforementioned node jbeing spread out around the input point of the node in a ‘star’arrangement;

FIG. 14 shows a scheme of a system in accordance with the presentinvention, based on a combination of the TDM and WDM techniques, andcapable of interrogating | sub-arrays of m nodes of n FOISs; the n FOISsof a node being spread out around the input point of the node in a‘star’ arrangement, the distance or fiber length between any two nodesof a sub-array being allowed to be of any value, and the distance orfiber length between any two sub-arrays being allowed to be of anyvalue;

FIG. 15 to 23 show the main characteristics of the fiber opticcomponents employed in the state of the art or/and in the presentinvention;

FIG. 15 describes the operation of a fiber optic circulator on anincident light; the component being employed in the state of the art andin the present invention;

FIG. 16 describes the operation on an incident light of a fiber opticcoupler of type <<1 input towards k outputs>>; the component beingemployed in the state of the art as a <<1 input towards 2 outputs>>coupler, and in the present invention as a <<1 input towards ≧2outputs>> coupler;

FIG. 17 a shows the reflection and transmission properties of a nonwavelength-selective mirror lit up by a broad band light source; thecomponent being employed in the state of the art and in the presentinvention;

FIG. 17 b shows the reflection and transmission properties of a FiberBragg Grating lit up by a broad band light source; the component beingemployed in the state of the art and in the present invention;

FIGS. 18 a, 18 b and 18 c show the reflection and transmissionproperties for some possible mirrors of the present invention.

FIG. 19 describes the operation of a wavelength demultiplexer of type<<1 input towards n outputs>> on a incident light comprising n specificwavelengths; the component being employed in the state of the art and inthe present invention;

FIG. 20 describes the operation of a wavelength-dropping component on anincident light comprising a number of specific wavelengths; thecomponent being employed in the state of the art;

FIG. 21 describes the operation of a wavelength band demultiplexer oftype <<1 input towards m outputs>> on an incident light comprising mspecific bands of wavelengths; the component being employed in thepresent invention;

FIG. 22 describes the operation of a wavelength band-dropping componenton an incident light comprising a number of specific bands ofwavelength; the component being employed in the present invention;

FIG. 23 describes the operation of an interleaver component; thecomponent being employed in the present invention;

FIG. 4 shows a scheme of a system in accordance with the presentinvention, based on a combination of the TDM and WDM techniques, andcapable of interrogating one group (or node) of n FOISs localized at thesame point. The proposed scheme is based on the state of the artprinciple (described in FIG. 1) of interrogation of one FOIS by a pulsedlight and it implements a new wavelength selective Time DivisionMultiplexing technique associated with a specific arrangement of FOISs,spread out around the input point of the group in a ‘star’ arrangement.The system comprises means 1 capable of generating a group 110 of nlight pulses in an optical fiber 13. Following the light path, thesystem also comprises

-   -   a reference interferometer 2 comprising a short path 2 a that        contains means 21 capable of phase modulating the optical field        that goes through it, and a long path 2 b; the optical path        difference between the short arm 2 a and long arm 2 b being        equal to D    -   a fiber optic circulator 4    -   a group 50 of n FOISs localized at the same point or node    -   a photo-detector 70    -   a demodulator 80

At the output of the reference interferometer 2, the group 110 of nlight pulses gives rise to the group 210 of n couples of light pulses;at the output of the group 50 of n FOISs spread out around the inputpoint of the group in a ‘star’ arrangement, the group 210 of n couplesof light pulses gives rise to the group 340 of n*2 couples of lightpulses.

FIG. 5 is a representation of the light sent in the telemetry fiber thatis required to interrogate the group (or node) of n FOISs localized atthe same point. This light forms the group 110 of n light pulsesP_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)), each of which having aspecific wavelength λ₀₁, λ₀₂, . . . , λ_(0n), and each of which beingdelayed one from another by a time t(D″) very closed to the time t(D).The group 110 is periodically generated by the means 1 at a repetitionrate Tr. On FIG. 5, the first repeated group of 110=P_(k)(λ₀₁),P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)) is referred as P_(k+1)(λ₀₁),P_(k+1)(λ₀₂), . . . , P_(k+1)(λ_(0n)). In the remaining part of thepresent description one will only take into account the group 110, thedescription being equivalent for the repeated groups of 110.

FIG. 5 a, 5 b, 5 c, 5 d show schemes of possible ways of constructionfor means 1; means 1 are capable of generating the group 110 of n lightpulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)), each pulse havinga specific wavelength, respectively λ₀₁, λ₀₂, . . . , λ_(0n) and eachpulse being delayed one from another by the time t(D″) very closed tot(D) (which is equal to the time propagation of light through theoptical path difference D in the reference interferometer 2).

The possible ways of construction for means 1 illustrated by FIGS. 5 a,5 b and 5 c first comprise n optical sources 601, 602, . . . , 60n whichemit respectively a light of wavelength λ₀₁, λ₀₂, . . . , λ_(0n)distinct one from another. A wavelength multiplexer 11 (of type <<ninputs towards 1 output), located at the output of the optical sourcesarray 601, 602, . . . , 60n combines the n lights of specific wavelengthλ₀₁, λ₀₂, . . . , λ_(0n) into an optical fiber 13. The wavelengthmultiplexer 11 is followed by an optical switch 120 on the optical fiber13. At the output of the wavelength multiplexer 11, the light is aContinuous Wave (CW) light comprising the n lights of differentwavelengths λ₀₁, λ₀₂, . . . , λ_(0n). The optical switch 120periodically pulses the CW light at its input to generate a light pulseat its output, P_(k)=P_(k)(λ₀₁, λ₀₂, . . . , λ_(0n)) repeated at arepetition rate Tr and containing the n wavelengths λ₀₁, λ₀₂, . . . ,λ_(0n). The generated light pulse P_(k)=P_(k)(λ₀₁, λ₀₂, . . . , λ_(0n))is then lead by the fiber 13 to means 14. The means 14 are capable ofgenerating, from the light pulse P_(k)=P_(k)(λ₀₁, λ₀₂, . . . , λ_(0n)),the group 110 made up of n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . ,P_(k)(λ_(0n)) of specific wavelengths respectively λ₀₁, λ₀₂, . . . ,λ_(0n). The n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n))are delayed one from another by the time t(D″) very closed to t(D) whichis equal to the time propagation of light through the optical pathdifference D in the reference interferometer 2.

The means 14 can be constructed in several ways: FIGS. 5 a, 5 b and 5 cillustrate three non restrictive examples.

In the way of construction illustrated by FIG. 5 a, the means 14comprise a fiber optic circulator 14 ₁ which couples the light out fromfiber 13 into a fiber 14 ₂. Fiber 14 ₂ comprises n mirrors 140 ₁, 140 ₂,. . . , 140 _(n) which selectively reflects, respectively the light ofwavelength λ₀₁, λ₀₂, . . . , λ_(0n) (and transmits all the non reflectedwavelengths). The mirrors are separated one from another on fiber 14 ₂by a round trip optical path that is equal to D″. The lights ofdifferent wavelengths that are reflected by the mirrors get back to thecirculator 14, that leads them to fiber 13. At the output of the means14, the light pulse P_(k)(λ₀₁, λ₀₂, . . . , λ_(0n)) gives rise to nlight pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)), each ofspecific wavelength respectively λ₀₁, λ₀₂, . . . , λ_(0n), the n lightpulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)) being delayed onefrom another by the time t(D″) corresponding to the time propagation oflight through the round trip optical path that separates to consecutivemirrors 140 _(i+1), 140 _(i) (1≦i≦n−1) on fiber 14 ₂. Mirrors 140 ₁, 140₂, . . . , 140 _(n) are preferentially Fiber Bragg Gratings.

In the way of construction illustrated by FIG. 5 b, the means 14comprise a fiber optic circulator 14 ₁ which couples the light out fromfiber 13 into a fiber 14 ₂, a wavelength demultiplexer 140 (of type <<ninputs towards 1 output>>) separating the n lights of wavelength λ₀₁,λ₀₂, λ₀₃, . . . , λ_(0n) contained in pulse P_(k)=P_(k)(λ₀₁, λ₀₂, λ₀₃, .. . , λ_(0n)) on n output fibers 142 ₁, 142 ₂, 142 ₃, . . . , 142 _(n).Each of the n fibers 142 ₁, 142 ₂, 142 ₃, . . . , 142 _(n) is terminatedby a mirror, respectively 140 ₁, 140 ₂, 140 ₃, . . . , 140 _(n); theaforementioned mirrors can be wavelength selective or not. The fiberoptic sections 142 _(i+1) (delimited by the corresponding output of thewavelength demultiplexer and mirror 140 _(i+1)) and 142 _(i) (delimitedby the corresponding output of the wavelength demultiplexer and mirror140 _(i)), where 1≦i≦n−1, form two optical path of round trip differenceD″. Each pulse light P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)), ofwavelength λ₀₁, λ₀₂, λ₀₃, . . . , λ_(0n), travels through its associatedfiber section, respectively 142 ₁, 142 ₂, 142 ₃, . . . , 142 _(n), andis reflected back by its associated mirror, respectively 140 ₁, 140 ₂,140 ₃, . . . , 140 _(n). Back to the wavelength demultiplexer 140, the nlight pulses are recombined onto the fiber 14 ₂ and lead by thecirculator 14 ₁ to the fiber 13. At the output of the circulator 14 ₁,the n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)) aredelayed one from another by the time t(D″) corresponding to the roundtrip difference of the optical paths formed by two consecutive fibersections 142 _(i), 142 _(i+1) where 1≦i≦n−1.

In the way of construction illustrated by FIG. 5 c, the means 14comprise, a wavelength separator 14 a of type <<1 input towards noutputs>>. From the light pulse P_(k)=P_(k)(λ₀₁, λ₀₂, λ₀₃, . . . ,λ_(0n)) made up of the n lights of wavelengths λ₀₁, λ₀₂, . . . , λ_(0n),the wavelength separator 14 a separate the n wavelengths λ₀₁, λ₀₂, . . ., λ_(0n) on n fibers respectively 142 ₁, 142 ₂, 142 ₃, . . . , 142 _(n)so as to form n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), P_(k)(λ₀₃), . . . ,P_(k)(λ_(0n)) on the n fibers, respectively 142 ₁, 142 ₂, 142 ₃, . . . ,142 _(n). The optical path difference between two consecutive fibersections 142 _(i) and 142 _(i+1) where 1≦i≦n−1, is equal to D″. Thewavelength combiner 14 b of type <<n inputs towards 1 output>> combinesthe n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), P_(k)(λ₀₃), . . . ,P_(k)(λ_(0n)) back onto fiber 13. At this stage, the n light pulsesP_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)) of specific wavelengthsλ₀₁, λ₀₂, . . . , λ_(0n) are delayed one from another by the time t(D″)and form the group 110 of light pulses. Means 14 a and 14 b can beachieved with a set of inter-leavers, or by a wavelength multiplexer anddemultiplexer respectively.

FIG. 5 d shows another possible way of construction for means 1. In thatscheme, n optical sources 601, 602, . . . , 60n emit each a light ofwavelength, respectively λ₀₁, λ₀₂, . . . , λ_(0n) distinct one fromanother. The n optical sources are each followed by an optical switch,respectively 121, 122, . . . , 12n, and by a wavelength multiplexer 11(of type <<n inputs towards 1 output) combining the output lights ofswitches 121, 122, . . . , 12n of wavelength respectively λ₀₁, λ₀₂, . .. , λ_(0n) onto the optical fiber 13. From the continuous wave lights ofwavelength λ₀₁, λ₀₂, . . . , λ_(0n), at their input, the opticalswitches 121, 122, . . . , 12n, periodically generate with a repetitionrate Tr, the light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n))respectively, delayed one from another by a time t(D″). Therefore, theoptical switches 121, 122, . . . , 12 _(n) need to be activatedsuccessively with a time delay equals to t(D″). The wavelengthmultiplexer 11 ‘n inputs to 1 output’, located at the output of the noptival switchesn allows to combine the nlight pulses P_(k)(λ₀₁),P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)) into the optical fiber 13 so as toform the group 110 of n light pulses delayed one from another by thetime t(D″).

FIG. 6 represents the group 210 of n light pulses resulting at theoutput of the reference interferometer 2, from the group 110 of n lightpulses at its input. More precisely, at the output of means 1, the group110 of n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . . , P_(k)(λ_(0n)) ofwavelength respectively λ₀₁, λ₀₂, . . . , λ_(0n) and delayed one fromanother by the time t(D″) are lead to the reference interferometer 2 bythe optical fiber. The reference interferometer 2 is made up of twooptical paths 2 a et 2 b of difference D, very closed to the opticalpath D″. Thus, each of the n light pulses P_(k)(λ₀₁), P_(k)(λ₀₂), . . ., P_(k)(λ_(0n)) entering the reference interferometer 2 gives rise atthe output of the interferometer, to the couple of light pulses,respectively, [P_(k,a)(λ₀₁), P_(k,b)(λ₀₁)], [P_(k,a)(λ₀₂),P_(k,b)(λ₀₂)], . . . , [P_(k,a)(λ_(0n)), P_(k,b)(λ_(0n))]. The twopulses of each of the n couples [P_(k,a)(λ₀₁), P_(k,b)(λ₀₁)],[P_(k,a)(λ₀₂), P_(k,b)(λ₀₂)], . . . , [P_(k,a)(λ_(0n)), P_(k,b)(λ_(0n))]are delayed one from another by a time t(D) corresponding to the timepropagation of light of the optical path difference D between the paths2 a and 2 b. Thus, at the output of the reference interferometer 2, thegroup 210 of n couples of light pulses [P_(k,a)(λ₀₁), P_(k,b)(λ₀₁)],[P_(k,a)(λ₀₂), P_(k,b)(λ₀₂)], . . . , [P_(k,a)(λ_(0n)), P_(k,b)(λ_(0n))]is formed, the n couples being delayed one from another by the timet(D″) very closed to t(D). The group 110 of n light pulses beingperiodically repeated with a repetition rate Tr, the group 210 isrepeated at the same repetition rate Tr. The reference interferometer 2can was represented as a Mach Zender interferometer but can also be aMichelson interferometer.

As represented by FIG. 4, the group 210 of n couples of light pulses[P_(k,a)(λ₀₁), P_(k,b)(λ₀₁)], [P_(k,a)(λ₀₂), P_(k,b)(λ₀₂)], . . . ,[P_(k,a)(λ_(0n)), P_(k,b)(λ_(0n))], resulting at the output of referenceinterferometer 2, is then driven by fiber optic 31 to a circulator 4. Itis then driven by fiber optic 32 to a group (or node) 50 of n fiberoptic interferometric sensors geographically located at the same point.

FIG. 7 represents the group 340 of n*2 couples of light pulses,resulting from the group 210 of n couples of light pulses, at the outputof a group 50 of n fiber optic interferometric sensors located at thesame point and set into a ‘star like arrangement’ around this point.FIGS. 8 a and 8 b show two modes of realization of such a ‘star likearrangement’ around a point of n fiber optic interferometric sensorsforming the group 50. According to these two modes of realization, the nfiber optic interferometric sensors are Michelson interferometers.

In the mode of realization presented by FIG. 8 a, the n fiber opticinterferometric sensors share the same short arm forming the shortoptical path of the interferometers. The shared short arm is made up ofthe fiber optic section 530 _(a) delimited by the input point E0 (of thegroup 50) and a reference mirror 540 _(a). In addition the ninterferometric sensors each comprise a long arm of its own forming thelong optical path of each interferometer. The n long arms of the ninterferometric sensors are each made up of the fiber optic section 520_(b) delimited by the input point E0 and point F0 (the fiber section 520_(b) is common to all the sensors), added to the distinct fiber opticsection for each of the n sensors respectively 530 _(b,1), 530 _(b,2), .. . , 530 _(b,n) delimited by point F0 and mirrors 540 _(b,1), 540_(b,2), . . . , 540 _(b,n) respectively. The n fiber opticinterferometric sensors comprise each a compliant body respectively 550₁, 550 ₂, . . . , 550 _(n), on which fiber optic sections 530 _(b,1),530 _(b,2), . . . , 530 _(b,n) are respectively arranged. The round tripoptical path difference between the long and short arms of each of the nfiber optic interferometric sensors of group 50 is equal to D′ and isvery closed to the optical path difference D of the referenceinterferometer 2. In the mode of realization presented by FIG. 8 a, then mirrors 540 _(b,1), 540 _(b,2), . . . , 540 _(b,n) can either beselective or not in wavelength. If the n mirrors 540 _(b,1), 540 _(b,2),. . . , 540 _(b,n) are wavelength selective, they preferentially areFiber Bragg Gratings that selectively reflects, respectively, wavelengthλ₀₁, λ₀₂, . . . λ_(0n). In the mode of realization presented by FIG. 8a, mirror 540 _(a) is a non wavelength selective mirror that reflectsall the wavelengths λ₀₁, λ₀₂, . . . , λ_(0n). The group 210 of n couplesof light pulses [P_(k,a)(λ₀₁), P_(k,b)(λ₀₁)], [P_(k,a)(λ₀₂),P_(k,b)(λ₀₂)], . . . , [P_(k,a)(λ_(0n)), P_(k,b)(λ_(0n))] entering thegroup 50 of n fiber optic interferometric sensors is driven by fiberoptic 32 to a fiber optic coupler 510 of type <<1 input towards 2outputs>>. The group 210 of n couples of light pulses is thus powerdivided into two groups 210 a and 210 b of n couples of light pulses,carried respectively by fiber 530 _(a) and 520 _(b). The group 210 _(a)is carried by the short optical path 530 _(a) shared by the n fiberoptic interferometric sensors, and reflected by the non wavelengthselective mirror 540 a; it is then recoupled into the optical fiber 32by the fiber optic coupler 510: back onto the fiber optic 32, the group210 a is made up of n couples of light pulses [P_(k,aa)(λ₀₁),P_(k,ba)(λ₀₁)], [P_(k,aa)(λ₀₂), P_(k,ba)(λ₀₂)], . . . ,[P_(k,aa)(λ_(0n)), P_(k,ba)(λ_(0n))], the n couples being time delayedone from another by the time t(D″), the 2 pulses of each couple beingtime delayed one from another by the time t(D), the times t(D″) beingvery closed to the time t(D). The group 210 b of n couples of lightpulses, formed onto the optical fiber 520 b, is driven to a wavelengthdemultiplexer 520 of type <<1 input towards n outputs>>. The wavelengthdemultiplexer 520 separates the group 210 b of n couples of pulses inton couples of pulses, respectively [P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)] ontothe optical fiber 530 _(b,1), [P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂)] onto theoptical fiber 530 _(b,2), . . . , [P_(k,ab)(λ_(0n)), P_(k,bb)(λ_(0n))]onto the optical fiber 530 _(b,n). The optical fibers 530 _(b,1), 530_(b,2), . . . , 530 _(b,n) are ended respectively by the mirrors 540_(b,1), 540 _(b,2), . . . , 540 _(b,n). The n couples of pulses[(P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)], [(P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂), . . . ,(P_(k,ab)(λ_(0n)), P_(k,bb)(λ_(0n))] are then reflected respectively bythe mirrors 540 _(b,1), 540 _(b,2), . . . , 540 _(b,n) and driven backto the wavelength demultiplexer 520 respectively by the optical fibers530 _(b,1), 530 _(b,2), . . . , 530 _(b,n). The wavelength demultiplexer520 (that works symmetrically regarding the light sense of propagation)combines the n couples of pulses [(P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)],[(P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂), . . . , [(P_(k,ab)(λ_(0n)),P_(k,bb)(λ_(0n))] back onto the optical fiber 520 b. The recombinedgroup of n couples of pulses [P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)],[P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂)], . . . , P_(k,ab)(λ_(0n)),P_(k,bb)(λ_(0n)) is then recoupled onto the optical fiber 32 by thefiber optic coupler 510. Back onto the optical fiber 32, the group 210 bis thus made up of n couples of pulses [P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)],[P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂)], . . . , [P_(k,ab)(λ_(0n)),P_(k,bb)(λ_(0n))], the n couples of pulses being time delayed one fromanother by the time t(D″), the 2 pulses of each couple being timedelayed one from another by the time t(D), t(D″) and being very closedto t(D). In addition, back onto the optical fiber 32, the two couples ofpulses [P_(k,aa)(λ₀₁), P_(k,ba)(λ₀₁)] and [P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)](respectively [P_(k,aa)(λ₀₂), P_(k,ba)(λ₀₂)] and [P_(k,ab)(λ₀₂),P_(k,bb)(λ₀₂)], . . . , [P_(k,aa)(λ_(0n)), P_(k,ba)(λ_(0n))] and[P_(k,ab)(λ_(0n)), P_(k,bb)(λ_(0n))]), from respectively groups 210 aand 210 b, are time delayed one from another by the time t(D″)corresponding to the time propagation of light through the round tripoptical path difference between the short and long arms of each of the nfiber optic interferometric sensors of group 50. Thus, at the output ofthe node 50 of n fiber optic interferometric sensors, the group 340 ofn*2 couples of pulses [P_(k,aa)(λ₀₁), P_(k,ba)(λ₀₁)], [P_(k,ab)(λ₀₁),P_(k,bb)(λ₀₁)], [P_(k,aa)(λ₀₂), P_(k,ba)(λ₀₂)], [P_(k,ab)(λ₀₂),P_(k,bb)(λ₀₂)], . . . , [P_(k,aa)(λ_(0n)), P_(k,ba)(λ_(0n))],[P_(k,ab)(λ_(0n)), P_(k,bb)(λ_(0n))], represented by FIG. 7, is formed:the 2 couples of pulses {[P_(k,aa)(λ₀₁), P_(k,ba)(λ₀₁)], [P_(k,ab)(λ₀₁),P_(k,bb)(λ₀₁)]}, and the 2 couples of pulses {[P_(k,aa)(λ_(0(i+1))),P_(k,ba)(λ_(0(i+1)))], [P_(k,ab)(λ_(0(i+1))), P_(k,bb)(λ_(0(i+1)))]},1≦i≦n, being time delayed one from another by the time t(D″), the 2couples of pulses [P_(k,aa)(λ_(0i)), P_(k,ba)(λ_(0i))] and[P_(k,ab)(λ_(0i)), P_(k,bb)(λ_(0i))] being time delayed one from anotherby the time t(D′), and the two pulses P_(k,aa)(λ_(0i)) andP_(k,ba)(λ_(0i)) of couple [P_(k,aa)(λ_(0i)), P_(k,ba)(λ_(0i))] andP_(k,ab)(λ_(0i)) and P_(k,bb)(λ_(0i)) of couple [P_(k,ab)(λ_(0i)),P_(k,bb)(λ_(0i))] being time delayed one from another by the time t(D),the times t(D), t(D′) and t(D″) being very closed. The group 110 oflight pulses being time repeated with a repetition rate Tr, the group340 of light pulses is time repeated at the same repetition rate.

In the mode of realization presented by FIG. 8 b, the n fiber opticinterferometric sensors each comprise the same short arm thatcorresponds to the fiber optic section 530 _(a) delimited by the pointG0 and the reference mirror 540 _(a); in addition, the n interferometricsensors comprise each a long arm of its own that correspondsrespectively to the fiber optic sections 530 _(b,1), 530 _(b,2), . . . ,530 _(b,n) delimited by the point G0 and respectively the mirrors 540_(b,1), 540 _(b,2), . . . , 540 _(b,n). The n fiber opticinterferometric sensors comprise each a compliant body respectively 550₁, 550 ₂, . . . , 550 _(n), on which fiber optic sections 530 _(b,1),530 _(b,2), . . . , 530 _(b,n) are respectively arranged. The round tripoptical path difference between the short and long arms of each of the nfiber optic interferometric sensors of group 50 equals D′ and is veryclosed to the difference D between the optical paths 2 a et 2 b ofreference interferometer 2. In the mode of realization presented by FIG.8 b, the n mirrors 540 _(b,1), 540 _(b,2), . . . , 540 _(b,n) arewavelength selective, and each selectively reflects, respectivelywavelength λ₀₁, λ₀₂, . . . , λ_(0n) Mirrors 540 _(b,1), 540 _(b,2), . .. , 540 _(b,n) are preferentially Fiber Bragg Gratings. The mirror 540_(a) is a non wavelength selective mirror that reflects all thewavelengths λ₀₁, λ₀₂, . . . λ_(0n). The group 210 of n couples of pulses[P_(k,a)(λ₀₁), P_(k,b)(λ₀₁)], [P_(k,a)(λ₀₂), P_(k,b)(λ₀₂)], . . . ,[P_(k,a)(λ_(0n)), P_(k,b)(λ_(0n))] entering the group 50 of n fiberoptic interferometric sensors is driven by the optical fiber 32 to afiber optic coupler 510 of type <<1 input towards (n+1) outputs>>. Thefiber optic coupler 510 divides the light power coming from the opticalfiber 32 onto (n+1) optical fibers 530 _(b,1), 530 _(b,2), . . . , 530_(b,n) and 530 a. The group 210 of n couples of pulses [P_(k,a)(λ₀₁),P_(k,b)(λ₀₁)], [P_(k,a)(λ₀₂), P_(k,b)(λ₀₂)], . . . , [P_(k,a)(λ_(0n)),P_(k,b)(λ_(0n))] is thus divided into (n+1) groups 210 _(b,1), 210_(b,2), . . . , 210 _(b,n) et 210 _(a) of n couples of pulses carried onrespectively by the optical fibers 530 _(b,1), 530 _(b,2), . . . , 530_(b,n) et 530 _(a), the (n+1) groups 210 _(b,1), 210 _(b,2), . . . , 210_(b,n) and 210 _(a) each comprising n couples of pulses [P_(k,a)(λ₀₁),P_(k,b)(λ₀₁)], [P_(k,a)(λ₀₂), P_(k,b)(λ₀₂)], . . . , [P_(k,a)(λ_(0n)),P_(k,b)(λ_(0n))]. The group 210 a is reflected by the non wavelengthsselective mirror 540 a, and coupled back onto the optical fiber 32 bythe fiber optic coupler 510: back onto the optical fiber 32, the group210 a is made up of n couples of pulses [P_(k,aa)(λ₀₁), P_(k,ba)(λ₀₁)],[P_(k,aa)(λ₀₂), P_(k,ba)(λ₀₂)], . . . , [P_(k,aa)(λ_(0n)),P_(k,ba)(λ_(0n))], the n couples of pulses being time delayed one fromanother by the time t(D″), the 2 pulses of each couple being timedelayed one from another by the time t(D), the time t(D″) being veryclosed to the time t(D). The group 210 _(b,1) (respectively 210 _(b,2),. . . , 210 _(b,n)) of n couples of pulses is reflected by mirror 540_(b,1) (respectively 540 _(b,2), . . . 540 _(b,n)) that selectivelyreflects the wavelength λ₀₁ (respectively λ₀₂, λ_(0n)): back from mirror540 _(b,1) (respectively 540 _(b,2), . . . , 540 _(b,n)), the group 210_(b,1) (respectively 210 _(b,2), . . . , 210 _(b,n)) is then made up ofthe unique couple of pulses [P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)] (respectively[P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂)], . . . , [P_(k,ab)(λ_(0n)),P_(k,bb)(λ_(0n))]), recoupled onto the optical fiber 32 by the fiberoptic coupler 510. Back onto the optical fiber 32, the groups 210_(b,1), 210 _(b,2), 210 _(b,n) form a group of n couples of pulses[P_(k,ab)(λ₀₁), P_(k,bb)(λ₀₁)], [P_(k,ab)(λ₀₂), P_(k,bb)(λ₀₂)], . . . ,[P_(k,ab)(λ_(0n)), P_(k,bb)(λ_(0n))], the n couples of pulses being timedelayed one from another by the time t(D″), the two pulses of eachcouple being time delayed one from another by the time t(D), the timet(D″) being very closed to the time t(D). Thus, at the output of thegroup 50 of n fiber optic interferometric sensors, the group 340 of n*2couples of pulses [P_(k,aa)(λ₀₁), P_(k,ba)(λ₀₁)], [P_(k,ab)(λ₀₁),P_(k,bb)(λ₀₁)], [P_(k,aa)(λ₀₂), P_(k,ba)(λ₀₂)], [P_(k,ab)(λ₀₂),P_(k,bb)(λ₀₂)], . . . , [P_(k,aa)(λ_(0n)), P_(k,ba)(λ_(0n))],[P_(k,ab)(λ_(0n)), P_(k,bb)(λ_(0n))], represented by FIG. 7, is formed:the 2 couples of pulses {[P_(k,aa)(λ_(0i)), P_(k,ba)(λ_(0i))],[P_(k,ab)(λ_(0i)), P_(k,bb)(λ_(0i))]}, and the 2 couples of pulses{[P_(k,aa)(λ_(0(i+1))), P_(k,ba)(λ_(0(i+1)))], [P_(k,ab)(λ_(0(i+1))),P_(k,bb)(λ_(0(i+1)))]}, 1≦i≦n, being time delayed one from another bythe time t(D″), the 2 couples of pulses [P_(k,aa)(λ_(0i)),P_(k,ba)(λ_(0i))] and [P_(k,ab)(λ_(0i)), P_(k,bb)(λ_(0i))] being timedelayed one from another by the time t(D′), and the two pulsesP_(k,aa)(λ_(0i)) and P_(k,ba)(λ_(0i)) of couple [P_(k,aa)(λ_(0i)),P_(k,ba)(λ_(0i))] and P_(k,ab)(λ_(0i)) and P_(k,bb)(λ_(0i)) of couple[P_(k,ab)(λ_(0i)), P_(k,bb)(λ_(0i))] being time delayed one from anotherby the time t(D), the times t(D), t(D′) and t(D″) being very closed. Thegroup 110 of light pulses being time repeated with a repetition rate Tr,the group 340 of light pulses is time repeated at the same repetitionrate.

In accordance with FIG. 4, the group 340 of n*2 couples of pulses isthen driven to the optical fiber 33 by the fiber optic circulator 4, andthen carried to the optical photo-detector 70. By construction, the 2pulses out of each of the n fiber optic interferometric sensors of group50, P_(k,ab)(λ_(0i)) and P_(k,ba)(λ_(0i)) (1≦i≦n ), get to thephoto-detector 70 at the same time, and the optical fields (born by thetwo pulses) can interfere together. The light pulse P_(k,ba)(λ_(0i))carried by the long optical path 2 b of reference interferometer 2 andthe short optical path of the fiber optic interferometric sensor i isnot subjected to any external perturbation, whereas pulseP_(k,ab)(λ_(0i)) carried by the short optical path 2 a of referenceinterferometer 2 and the long optical path of the fiber opticinterferometric sensor i is subjected to the external perturbations.Also, by construction and thanks to

-   -   means 1 ensuring the time separation of the wavelengths λ_(0i),        1≦i≦n at the optical sources level    -   the group 50 of n fiber optic interferometric sensors located at        the same point and set into a ‘star’ like arrangement around the        input point of the group 50, each sensor i of the group 50 being        associated with a distinct wavelength λ_(0i)        of the present invention, the n fiber interferometric signals        resulting from the n fiber optic sensors are time separated,        without the need to add fiber optic sections of accurate length        in between any two sensors, or without making use of the long        sensitive arms of the sensors. The separation of the n        interferometric signals from the in fiber optic interferometric        sensors can thus be achieved through the time gating of the        pulses. The demodulator 80 at the output of photo-detector 70        allows to continuously compute the phase difference between the        optical fields born by pulses P_(k,ab)(λ_(0i)) and        P_(k,ba)(λ_(0i)) (1≦i≦n), and thus to evaluate the external        perturbations locally applied on each fiber optic        interferometric sensor.

The system described above shows several advantages. Indeed the timedelays in between the n interferometric signals of a group of ntime-multiplexed interferometric sensors are generated by the means 1 atthe level of the optical sources, and not at the level of the sensorsthemselves in accordance to previous art systems. In addition, the‘star’ like arrangement of the n fiber interferometric sensors of group50 doesn't affect the time delays generated by the means 1. Thus, on thecontrary to the previous art systems, there is no need to add fibersections of accurate length in between any two sensors, reducing thesystem building costs and increasing its reliability; the fiber opticsections forming the long arm and sensitive part of each sensor are notused neither, reducing the Cross Talk between the multiplexed sensors.

FIG. 9 is a schematic of a system in accordance with the presentinvention, increasing the multiplexing density of the system describedby FIG. 4 to m nodes (or groups) of n interferometric sensors each, andin which, the n fiber optic sensors are located at the same point; the mnodes of the system can be spaced by any fiber length. The means 1generate the m groups 111, 112, . . . , 11j, . . . , 11m of light pulsesdedicated to the interrogation respectively of the m groups (or nodes)51, 52, . . . , 5j, . . . , 5m of n sensors each. The m groups 111, 112,. . . , 11j, . . . , 11m of pulses are driven by the optical fiber 13 toa reference interferometer 2, of which the short arm 2 a comprises means21 capable of actively phase modulating the optical field that goesthrough it, and of which the optical path difference between long andshort arms equals D. The means 1 and reference interferometer 2 formtogether the interrogation system S of sub-array 90, made up of m nodesof n sensors each. At the output of reference interferometer 2, the mgroups 111, 112, . . . , 11j, . . . , 11m of light pulses give rise to mgroups 211, 212, . . . , 21j, . . . , 21m of light pulses that aredriven by the optical fiber 31 to circulator 4, and directed by thelatest onto the optical fiber 32. The optical fiber 32 enters the mgroups 51, 52, . . . , 5j, . . . , 5m of n sensors. The m groups 51, 52,. . . , 5j, . . . , 5m are successively spread onto the optical fiber32, and respectively separated by the optical fiber sections 61 a, 62 a,. . . , 6ja, . . . , 6jm. These fiber optic sections can be of anylength and don't have to be equal in length. The means 61, 62, . . . ,6j, . . . , 6m selectively extracts, respectively the wavelength bandB₁, B₂, . . . , B_(j), . . . , B_(m) towards, respectively the group 51,52, . . . , 5m of n fiber optic sensors. Thus, the group 211(respectively 212, . . . , 21j, . . . , 21m) of pulses is carried to thegroup 51 (respectively 52, . . . , 5j, . . . , 5m) of sensors, at theoutput of which it gives rise to the group 341 of pulses (respectively342, . . . , 34j, . . . , 34m). Group 341 (respectively 342, . . . ,34j, . . . , 34m) is recombined back onto the optical fiber 32 by thecomponent 61 (respectively 62, . . . , 6j, . . . , 6m). The m groups341, 342, . . . , 34j, . . . , 34m are driven by the optical fiber 32 tothe circulator 4 that drives them to the optical fiber 33 and to a mwavelength bands demultiplexer 40 of type <<1 input towards m outputs>>.Demultiplexer 40 separates each of the m wavelength bands B₁, B₂, . . ., . . . , B_(j), . . . , B_(m) respectively onto the m output opticalfibers 33 ₁, 33 ₂, . . . , 33 _(j), . . . , 33 _(m). Thus the m groupsof pulses 341, 342, . . . , 34j, . . . , 34m are each drivenrespectively to the photo-detectors 71, 72, . . . , 7j, . . . , 7m,followed respectively by demodulators 81, 82, . . . , 8j, . . . , 8m.

FIG. 10 is a representation of the light coupled into the telemetryoptical fiber 13, to interrogate the m groups (or nodes) 51, 52, . . . ,5j, . . . , 5m, each of the m groups 5j being made up of n fiber opticinterferometric sensors located at the same point and set in a ‘star’like arrangement around the input point of the group 5j. The lightcoupled into the optical fiber 13 is made up of the m groups 111, 112, .. . , 11j, . . . , 11m, each of the m groups 11j being made up of nlight pulses P_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) ofspecific wavelength respectively λ_(j1), λ_(j2), . . . , λ_(jn). The nlight pulses P_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) aretime delayed one from another by the time t(D″) very closed to the timet(D) of light propagation through the optical path D. The means 1periodically generate the m groups 111, 112, . . . , 11j, . . . , 11m ofn light pulses with a repetition rate Tr.

FIGS. 10 a, 10 b, 10 c and 10 d show different modes of realization formeans 1, capable of generating the m groups 111, 112, . . . , 11j, . . ., 11m, each of the m groups 11j, 1≦j≦m, being made up of n light pulsesP_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) of specificwavelength respectively λ_(j1), λ_(j2), . . . , λ_(jn), the n lightpulses being delayed one from another by the time t(D″) very closed tothe time t(D) of light propagation through the optical path difference Din the reference interferometer 2.

The modes of realization of means 1 shown by FIGS. 10 a, 10 b and 10 ccomprise m*n optical sources forming m groups S₁=(611, 612, . . . ,61n), S₂=(621, 622, . . . , 62n), . . . , S_(j)=(6j1, 6j2, . . . , 6jn),. . . , S_(m)=(6m1, 6m2, . . . , 6mn). The m*n sources emit a continuouswave light of specific wavelength respectively λ₁₁, λ₁₂, . . . , λ_(1n),λ₂₁, λ₂₂, . . . λ_(2n), . . . , λ_(j1), λ_(j2), . . . , λ_(jn), . . . ,λ_(m1), λ_(m2), . . . , λ_(mn) distinct one from another. The m groupsS₁, S₂, . . . , S_(j), . . . S_(m) form m wavelength bands, respectivelyB₁=(λ₁₁, λ₁₂. . . , λ_(1n)), B₂=(λ₂₁, λ₂₂, . . . , λ_(2n)), . . . ,B_(j)=(λ_(j1), λ_(j2), . . . , λ_(jn)), . . . , B_(m)=(λ_(m1), λ_(m2), .. . , λ_(mn)). At the output of the m*n optical sources (611, 612, . . ., 61n), (621, 622, . . . , 62n), . . . , (6j1, 6j2, . . . , 6jn) , . . ., (6m1, 6m2, . . . , 6mn), a wavelength multiplexer 11 combines the m*nlights of specific wavelength λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . ., λ_(2n), . . . , λ_(j1), λ_(j2), . . . , λ_(jn), . . . , λ_(m1),λ_(m2), . . . , λ_(mn) onto the optical fiber 13; the continuous wavelight that comprises the m*n wavelengths λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁,λ₂₂, . . . , λ_(2n), . . . , λ_(j1), λ_(j2), . . . , λ_(jn), . . . ,λ_(m1), λ_(m2), . . . , λ_(mn) is carried to an optical switch 120. Thelatest periodically generates a light pulse P_(k)(λ₁₁, λ₁₂, . . . ,λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(j1), λ_(j2), . . . ,λ_(jn), . . . , λ_(m1), λ_(m2), . . . , λ_(mn)), spectrally made up ofthe m*n wavelengths λ₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ₂n , . .. , λ_(j1), λ_(j2), . . . , λ_(jn), . . . , λ_(m1), λ_(m2), . . . ,λ_(mn). The light pulse P_(k)(λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . ., λ_(2n), . . . , λ_(j1), λ_(j2), . . . , λ_(jn), . . . , λ_(m1),λ_(m2), . . . , λ_(mn)) resulting at the output of the optical switch120, is thus driven to means 14 by the optical fiber 13. From a lightpulse P_(k)(λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . ,λ_(j1), λ_(j2), . . . , λ_(jn), . . . , λ_(m1), λ_(m2), . . . , λ_(mn)),spectrally made up of the m*n wavelengths λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁,λ₂₂, . . . , λ_(2n), . . . , λ_(j1), λ_(j2), . . . , λ_(jn), . . . ,λ_(m1), λ_(m2), . . . , λ_(mn), the means 14 are capable of generatingthe m groups of pulses 111, 112, . . . , 11j, . . . , 11m, each of the mgroups 11j, 1≦j≦m, being made up of n light pulses P_(k)(λ_(j1)),P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) of specific wavelength respectivelyλ_(j1), λ_(j2), . . . , λ_(jn), the n pulses being delayed one fromanother by a time t(D″) very closed to the time t(D).

Means 14 can be achieved among different modes of realization describedby FIGS. 10 a, 10 b and 10 c.

In the mode of realization of FIG. 10 a, the means 14 comprise a fiberoptic circulator 14 ₁ that drives the light from the optical fiber 13 tothe optical fiber 14 ₂. The optical fiber 14 ₂ comprises m*n mirrors 141₁, 141 ₂, . . . , 141 _(n), 142 ₁, 142 ₂, . . . , 142 _(n), . . . ,14j₁, 14j₂, . . . , 14j_(n), . . . , 14m₁, 14m₂, . . . , 14m_(j), . . ., 14m_(n) that selectively reflect respectively the wavelength λ₁₁, λ₁₂,. . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(j1), λ_(j2), . . .,λ_(jn), . . . , λ_(m1), λ_(m2), . . . , λ_(mn). The m*n mirrors areFiber Bragg Gratings. The m*n mirrors are gathered into n groups G₁=(141₁, 142 ₁, . . . , 14j₁, . . . , 14m₁), G₂=(141 ₂, 142 ₂, . . . , 14j₂, .. . , 14m₂), . . . G_(n)=(141 _(n), 142 _(n), . . . , 14j_(n), . . . ,14m_(n)), each group containing m mirrors successively written along theoptical fiber 14 ₂. For 1≦j≦m, the n mirrors 14j₁, 14j₂, . . . , 14j_(n)are distant one from another by a single trip optical path equal toD″/2. The light reflected by the mirrors gets back to circulator 14 ₁,and to the optical fiber 13. At the output of means 14, the light pulseP_(k)(λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . ,λ_(j1), λ_(j2), . . . , λ_(jn), . . . , λ_(m1), λ_(m2), . . . , λ_(mn))gives rise to the m groups 111, 112, . . . , 11j, . . . , 11m, each ofthe m groups 11j being made up of n pulses P_(k)(λ_(j1)), P_(k)(λ_(j2)),. . . , P_(k)(λ_(jn)) of specific wavelength respectively λ_(j1),λ_(j2), . . . , λ_(jn), the n light pulses P_(k)(λ_(j1)), P_(k)(λ_(j2)),. . . , P_(k)(λ_(jn)) being time delayed one from another by the timet(D″) corresponding to the time propagation of light through the roundtrip between two successive mirrors 14j_(i+1), 14j_(i) (1≦i≦n−1).

In the mode of realization of FIG. 10 b, the means 14 comprise

-   a fiber optic circulator 14, that drives the light from the optical    fiber 13 to the optical fiber 14 ₂,-   a wavelength band demultiplexer 14 ₃ of type <<1 input towards m    outputs>> that separates the m bands B₁, B₂, . . . , B_(j), . . . ,    B_(m) at its input, onto m optical fibers, respectively 14 _(2,1),    14 _(2,2), . . . , 14 _(2,j), . . . , 14 _(2,m). Each of the m    optical fibers 14 _(2,1), 14 _(2,2), . . . , 14 _(2,j), . . . , 14    _(2,m) is ended by means respectively 161, 162, . . . , 16j, . . . ,    16m. The means 161, 162, . . . , 16j, . . . , 16m are built in    accordance with the means 160 described in FIG. 5 b. More precisely,    the means 16j, 1≦j≦m, comprise an optical fiber 14 _(2,j) and a    wavelength demultiplexer 14j of type <<1 input towards n outputs>>    separating the n wavelengths λ_(j1), λ_(j2), λ_(j3), . . . , λ_(jn)    of the light pulse P_(k)=P_(k)(λ_(j1), λ_(j2), λ_(j3), . . . ,    λ_(jn)) onto n optical fibers 142 _(j1), 142 _(j2), 142 _(j3), . . .    , 142 _(jn). Each of the n optical fibers 142 _(j1), 142 _(j2), 142    _(j3), . . . , 142 _(jn) is ended by a mirror, respectively 14j₁,    14j₂, 14j₃, . . . , 14j_(n), selective or not in wavelength. In    addition, the fiber optic section 142 _(j,(i+1)) (between the output    of the demultiplexer 14 _(j) and mirror 14j_(i+1)) and the fiber    optic section 142 _(j,i) (between the output of the demultiplexer    14j and mirror 14j_(i)) where 1≦i≦n−1 form optical paths of    difference D″/2.

In the mode of realization of FIG. 10 c, the means 14 comprise

-   means 14 a separating the m*n wavelengths of the output light of the    optical switch 120 into n packets of m wavelengths each (λ₁₁, λ₂₁, .    . . , λ_(m1)), (λ₁₂, λ₂₂, . . . , λ_(m2)), (λ₁₃, λ₂₃, . . . ,    λ_(m3)), . . . , (λ_(1n), λ_(2n), . . . , λ_(mn)), respectively onto    the n optical fibers 142 ₁, 142 ₂, 142 ₃, . . . , 142 _(n),-   means 14 b re-combining the n packets of wavelengths onto the    optical fiber 13.    In addition, the difference of the two optical paths formed by the    two fiber sections 142 _(i) and 142 _((i+1)), 1≦i≦n−1, is equal to    D″. Means 14 a and 14 b can be formed by a set of interleavers.

The means 1 can be achieved among the mode of realization described byFIGS. 10 d where they comprise

-   m*n optical source emitting a continuous wave light of specific    wavelength (λ₁₁, λ₂₁, . . . , λ_(m1), λ₁₂, λ₂₂, . . . , λ_(m2), . .    . , λ_(1n), λ_(2n), . . . , λ_(mn)),-   a set of n wavelength multiplexers 411, 412, . . . , 41i, . . . ,    41n of type <<m inputs towards one output>>, each of the n    multiplexers 41i, 1≦i≦n, combining the m wavelengths λ_(1i), λ_(2i),    . . . , λ_(mi) onto an optical switch 12i,-   n optical switches 121, 122, . . . , 12i, . . . , 12n that generates    the n pulses of light respectively, P_(k)(λ₁₁, λ₂₁, . . . , λ_(m1)),    P_(k)(λ₁₂, λ₂₂, . . . , λ_(m2)), . . . , P_(k)(λ_(1i), λ_(2i), . . .    , λ_(mi)), . . . , P_(k)(λ_(1n), λ_(2n), . . . , λ_(mn)) time    delayed one from another by the time t(D″),-   means 11 that combine the n light pulses P_(k)(λ₁₁, λ₂₁, . . . ,    λ_(m1)), P_(k)(λ₁₂, λ₂₂, . . . , λ_(m2)), . . . , P_(k)(λ_(1i),    λ_(2i), . . . , λ_(mi)), . . . , P_(k)(λ_(1n), λ_(2n), . . . ,    λ_(mn)) onto the optical fiber 13. The m groups 111, 112, . . . ,    11j, . . . , 11m are thus formed onto the optical fiber 13. The    means 11 can be built with a set of interleavers.

FIG. 11 represents the group 21j (1≦j≦m) of n couples of light pulsesresulting at the output of reference interferometer 2, from the group11j of n light pulses. More precisely, at the output of means 1, thegroup 11j of n light pulses P_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . ,P_(k)(λ_(jn)) of wavelength respectively λ_(j1), λ_(j2), . . . , λ_(jn)and delayed one from another by the time t(D″), is driven by the opticalfiber 13 to the reference interferometer 2. The reference interferometer2 is formed by the two optical paths 2 a and 2 b of difference D, veryclosed to the optical path D″. Thus, each of the n pulses P_(k)(λ_(j1)),P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) entering the referenceinterferometer 2 gives rise at its output to a couple of pulses,respectively [P_(k,a)(λ_(j1)), P_(k,b)(λ_(j1))], [P_(k,a)(λ_(j2)),P_(k,b)(λ_(j2))], . . . , [P_(k)(λ_(jn)), P_(k,b)(λ_(jn))]. The twopulses of each of the n couples [P_(k,a)(λ_(j1)), P_(k,b)(λ_(j1))],[P_(k,a)(λ_(j2)), P_(k,b)(λ_(j2))], . . . , [P_(k,a)(λ_(jn)),P_(k,b)(λ_(jn))] are time delayed one from another by the timedifference t(D) corresponding to the time propagation of light throughthe optical path difference between paths 2 a et 2 b. Thus, at theoutput of reference interferometer 2, the group 21j made up of n couplesof light pulses [P_(k,a)(λ_(j1)), P_(k,b)(λ_(j1))], [P_(k,a)(λ_(j2)),P_(k,b)(λ_(j2))], . . . , [P_(k,a)(λ_(jn)), P_(k,b)(λ_(jn))] is formed,the two pulses of each of the n couples being delayed one from anotherby the time t(D), the n couples being delayed one from another by thetime t(D″) very closed to t(D).

FIG. 12 represents the group 34j (1≦j≦m) of n*2 couples of light pulses,resulting from the group 21j of n couples of pulses at the output of agroup 5j of n fiber optic interferometric sensors located at the samepoint and set into a ‘star’ like arrangement around the input point ofthe group 5j.

The group 5j (1≦j≦m) is similar to the group 50 and can thus be built inaccordance with the two modes of realization for the group 50 describedby FIGS. 8 a and 8 b. One mode of realization for the group 5j isdescribed in FIG. 13; it is similar to that described by FIG. 8 a forthe building of group 50. More precisely, in the mode of realizationpresented by FIG. 13, the n fiber optic interferometric sensors sharethe same short arm forming the short optical path of theinterferometers. The shared short arm is made up of the fiber opticsection 53j_(a) delimited by the input point Ej (of the group 5j) and areference mirror 54j_(a). In addition the n interferometric sensors eachcomprise a long arm of its own forming the long optical path of eachinterferometer. The n long arms of the n interferometric sensors areeach made up of the fiber optic section 520 _(b) delimited by the inputpoint Ej and point Fj (the fiber section 52j_(b) is common to all thesensors), added to the distinct fiber optic section for each of the nsensors respectively 53j_(b,1), 53i_(b,2), . . . , 53j_(b,n) delimitedby point Fj and mirrors 54j_(b,1), 54j_(b,2), . . . , 54j_(b,n)respectively. The n fiber optic interferometric sensors comprise each acompliant body respectively 55j₁, 55j₂, . . . , 55j_(n), on which fiberoptic sections 53j_(b,1), 53j_(b,2), . . . , 53j_(b,n) are respectivelyarranged. The round trip optical path difference between the long andshort arms of each of the n fiber optic interferometric sensors of group5j is equal to D′ and is very closed to the optical path difference D ofthe reference interferometer 2. In the mode of realization presented byFIG. 13, the n mirrors 54j_(b,1), 54j_(b,2), . . . , 54j_(b,n) caneither be selective or not in wavelength. If the n mirrors 54j_(b,1),54j_(b,2), . . . , 54j_(b,n) are wavelength selective, theypreferentially are Fiber Bragg Gratings that selectively reflects,respectively, wavelength λ_(j1), λ_(j2), . . . , λ_(jn). In the mode ofrealization presented by FIG. 13, mirror 54j_(a) is a non wavelengthselective mirror that reflects all the wavelengths λ_(j1), λ_(j2), . . ., λ_(jn). The group 21j of n couples of light pulses [P_(k,a)(λ_(j1)),P_(k,b)(λ_(j1))], [P_(k,a)(λ_(j2)), P_(k,b)(λ_(j2))], . . . ,[P_(k,a)(λ_(jn)), P_(k,b)(λ_(jn))] entering the group 5j of n fiberoptic interferometric sensors is driven by fiber optic 32 to a fiberoptic coupler 51j of type <<1 input towards 2 outputs>>. The group 21jof n couples of light pulses is thus power divided into two groups 21jaand 21jb of n couples of light pulses, carried respectively by fiber53j_(a) and 52i_(b). The group 21ja is carried by the short optical path53j_(a) shared by the n fiber optic interferometric sensors, andreflected by the non wavelength selective mirror 54ja; it is thenrecoupled into the optical fiber 32 by the fiber optic coupler 51j: backonto the fiber optic 32, the group 21ja is made up of n couples of lightpulses [P_(k,aa)(λ_(j1)), P_(k,ba)(λ_(j1))], [P_(k,aa)(λ_(j2)),P_(k,ba)(λ_(j2))], . . . , [P_(k,aa)(λ_(jn)), P_(k,ba)(λ_(jn))], the ncouples being time delayed one from another by the time t(D″), the 2pulses of each couple being time delayed one from another by the timet(D), the times t(D″) being very closed to the time t(D). The group 21jbof n couples of light pulses, formed onto the optical fiber 52jb, isdriven to a wavelength demultiplexer 52j of type <<1 input towards noutputs>>. The wavelength demultiplexer 52j separates the group 21jb ofn couples of pulses into n couples of pulses, respectively[P_(k,ab)(λ_(j1)), P_(k,bb)(λ_(j1))] onto the optical fiber 53j_(b,1),[P_(k,ab)(λ_(j2)), P_(k,bb)(λ_(j2))] onto the optical fiber 53j_(b,2), .. . , [P_(k,ab)(λ_(jn)), P_(k,bb)(λ_(jn))] onto the optical fiber53j_(b,n). The optical fibers 53j_(b,1), 53j_(b,2), . . . , 53j_(b,n)are ended respectively by the mirrors 54j_(b,1), 54j_(b,2), . . . ,54j_(b,n). The n couples of pulses [(P_(k,ab)(λ_(j1)),P_(k,bb)(λ_(j1))], [(P_(k,ab)(λ_(j2)), P_(k,bb)(λ_(j2)), . . . ,[(P_(k,ab)(λ_(jn)), P_(k,bb)(λ_(jn))] are then reflected respectively bythe mirrors 54j_(b,1), 54j_(b,2), . . . , 54j_(b,n) and driven back tothe wavelength demultiplexer 52j respectively by the optical fibers53j_(b,1), 53j_(b,2), . . . , 53j_(b,n). The wavelength demultiplexer52j (that works symmetrically regarding the light sense of propagation)combines the n couples of pulses [(P_(k,ab)(λ_(j1)), P_(k,bb)(λ_(j1))],[(P_(k,ab)(λ_(j2)), P_(k,bb)(λ_(j2)), . . . , [(P_(k,ab)(λ_(jn)),P_(k,bb)(λ_(jn))] back onto the optical fiber 52jb. The recombined groupof n couples of pulses [P_(k,ab)(λ_(j1)), P_(k,bb)(λ_(j1))],[P_(k,ab)(λ_(j2)), P_(k,bb)(λ_(j2))], . . . , P_(k,ab)(λ_(jn)),P_(k,bb)(λ_(jn)) is then recoupled onto the optical fiber 32 by thefiber optic coupler 51j. Back onto the optical fiber 32, the group 21jbis thus made up of n couples of pulses [P_(k,ab)(λ_(j1)),P_(k,bb)(λ_(j1))], [P_(k,ab)(λ_(j2)), P_(k,bb)(λ_(j2))], . . . ,[P_(k,ab)(λ_(jn)), P_(k,bb)(λ_(jn))], the n couples of pulses being timedelayed one from another by the time t(D″), the 2 pulses of each couplebeing time delayed one from another by the time t(D), t(D″) and beingvery closed to t(D). In addition, back onto the optical fiber 32, thetwo couples of pulses [P_(k,aa)(λ_(j1)), P_(k,ba)(λ_(j1))] and[P_(k,ab)(λ_(j1)), P_(k,bb)(λ_(j1))] (respectively [P_(k,aa)(λ_(j2)),P_(k,ba)(λ_(j2))] and [P_(k,ab)(λ_(j2)), P_(k,bb)(λ_(j2))], . . . ,[P_(k,aa)(λ_(jn)), P_(k,ba)(λ_(jn))] and [P_(k,ab)(λ_(jn)),P_(k,bb)(λ_(jn))]), from respectively groups 21ja and 21jb, are timedelayed one from another by the time t(D′) corresponding to the timepropagation of light through the round trip optical path differencebetween the short and long arms of each of the n fiber opticinterferometric sensors of group 5j.

Thus, at the output of the node 5j of n fiber optic interferometricsensors, the group 34j of n*2 couples of pulses [P_(k,aa)(λ_(j1)),P_(k,ba)(λ_(j1))], [P_(k,ab)(λ_(j1)), P_(k,bb)(λ_(j1))],[P_(k,aa)(λ_(j2)), P_(k,ba)(λ_(j2))], [P_(k,ab)(λ_(j2)),P_(k,bb)(λ_(j2))], . . . , [P_(k,aa)(λ_(jn)), P_(k,ba)(λ_(jn))],[P_(k,ab)(λ_(jn)), P_(k,bb)(λ_(jn))], represented by FIG. 12, is formed:the 2 couples of pulses {[P_(k,aa)(λ_(ji)), P_(k,ba)(λ_(ji))],[P_(k,ab)(λ_(ji)), P_(k,bb)(λ_(ji))]}, and the 2 couples of pulses{[P_(k,aa)(λ_(j(i+1))), P_(k,ba)(λ_(j(i+1)))], [P_(k,ab)(λ_(j(i+1))),P_(k,bb)(λ_(j(i+1)))]}, 1≦i≦n, being time delayed one from another bythe time t(D″), the 2 couples of pulses [P_(k,aa)(λ_(ji)),P_(k,ba)(λ_(ji))] and [P_(k,ab)(λ_(ji)), P_(k,bb)(λ_(ji))] being timedelayed one from another by the time t(D′), and the two pulsesP_(k,aa)(λ_(ji)) and P_(k,ba)(λ_(ji)) of couple [P_(k,aa)(λ_(ji)),P_(k,ba)(λ_(ji))] and P_(k,ab)(λ_(ji)) and P_(k,bb)(λ_(ji)) of couple[P_(k,ab)(λ_(ji)), P_(k,bb)(λ_(ji))] being time delayed one from anotherby the time t(D), the times t(D), t(D′) and t(D″) being very closed. Thegroup 11j of light pulses being time repeated with a repetition rate Tr,the group 34j of light pulses is time repeated at the same repetitionrate.

By construction, the 2 pulses out of each of the fiber opticinterferometric sensor i (1≦i≦n) of group 5j (1≦j≦m), P_(k,ab)(λ_(ji))and P_(k,ba)(λ_(ji)), get to the photo-detector 7j at the same time, andthe optical fields (born by the two pulses) can interfere together. Thelight pulse P_(k,ba)(λ_(ji)) carried by the long optical path 2 b ofreference interferometer 2 and the short optical path of the fiber opticinterferometric sensor i of group 5j is not subjected to any externalperturbation, whereas pulse P_(k,ab)(λ_(ji)) carried by the shortoptical path 2 a of reference interferometer 2 and the long optical pathof the fiber optic interferometric sensor i is subjected to the externalperturbations. Also, by construction and thanks to

-   -   means 1 ensuring the time separation of the wavelengths λ_(ji),        1≦i≦n at the optical sources level    -   the group 5j of n fiber optic interferometric sensors located at        the same point and set into a ‘star’ like arrangement around the        input point Ej of the group 5j, each sensor i of the group 5j        being associated with a distinct wavelength λ_(ji)        of the present invention, the n fiber interferometric signals        resulting from the n fiber optic sensors are time delayed on the        photo-detector 7j, without the need to add fiber optic sections        of accurate length in between any two sensors, or without making        use of the long sensitive arms of the sensors. The separation of        the n interferometric signals from the in fiber optic        interferometric sensors can thus be achieved through the time        gating of the pulses. The demodulator 8j at the output of        photo-detector 7j allows to continuously compute the phase        difference between the optical fields born by pulses        P_(k,ab)(λ_(ji)) and P_(k,ba)(λ_(ji)) (1≦i≦n), and thus to        evaluate the external perturbations locally applied on each        fiber optic interferometric sensor of the group 5j.

As for the separation of the interferometric signals of theinterferometers of any two different groups, it is achieved bywavelength band demultiplexing through the means 61, 62, . . . , 6j.

The advantages of the system shown by FIG. 9 are similar to those of thesystem described by FIG. 4. Indeed the time delays in between the ninterferometric signals of a group of n time-multiplexed interferometricsensors are generated by means 1 at the level of the optical sources,and not at the level of the sensors themselves in accordance to previousart systems. In addition, the ‘star’ like arrangement of the n fiberinterferometric sensors of a group 5j, 1≦j≦m, doesn't affect the timedelays generated by the means 1. Thus, on the contrary to the previousart systems, there is no need to add fiber sections of accurate lengthin between any two sensors, reducing the system building costs andincreasing its reliability; the fiber optic sections forming the longarm and sensitive part of each sensor are not used neither, reducing theCross Talk between the multiplexed sensors. Besides, groups 51, 52, . .. , 5m being multiplexed in wavelength band, their distribution alongthe optical fiber 32 can be any: any two groups can be distant by anyfiber length.

The interrogation system S, comprising means 1 and referenceinterferometer 2, can be efficiently shared by several sub arrays, eachmade up of m groups of n sensors. FIG. 14 is a schematic of a system inaccordance with the present invention, increasing the multiplexingdensity of the system described by FIG. 9 to | sub arrays, each made upof m nodes (or groups) of n interferometric sensors, and in which, the nfiber optic sensors of a same node are located at the same point, the mnodes of each of the | sub arrays can be spaced by any fiber length, andthe distribution of the | sub arrays of the system can be any. Such asystem comprises a single interrogation system S described in FIG. 9, ofwhich light is coupled to a fiber optic coupler 310 of type <<1 inputtowards | outputs>>. The fiber optic coupler 310 divides the light powerout from the interrogation system S, onto | optical fibers 311, 312, . .. , 31r, . . . , 31|. The latest feed | sub arrays respectively 91, 92,. . . , 9y, . . . 9|, each made up of m nodes of n sensors. The subarrays 91, 92, . . . , 9y, . . . 9| are similar to the sub array 90shown in FIG. 9.

The advantages of the system shown by FIG. 14 are similar to those ofthe system described by FIG. 9. Indeed the time delays in between the ninterferometric time multiplexed signals, of a group 5j (1≦j≦m) of subarray y (1≦y≦l), are generated by means 1 at the level of the opticalsources, and not at the level of the sensors themselves in accordance toprevious art systems. In addition, the ‘star’ like arrangement of the nfiber interferometric sensors of a group 5j, 1≦j≦m, doesn't affect thetime delays generated by the means 1. Thus, whereas the previous artsystems multiplexing |*m*n sensors require to add |*m*n fiber sectionsof accurate length in between any two sensors, in the system describedby FIG. 14 and proposed here, this fabrication constraint doesn't hold;more precisely, the time delays between the time multiplexedinterferometric signals from the sensors are generated one for all atthe level of the means 1. The fiber optic sections forming the long armand sensitive part of each sensor are not used neither, reducing theCross Talk between the multiplexed sensors.

FIG. 15 shows a fiber optic circulator. This component is used in theprevious art systems and in the present invention. In a fiber opticcirculator, the light propagation is possible only from point A to pointB and from point B to point C. The light propagation from point B topoint A and from point B to point C is not permitted. This componentoperates non selectively regarding the wavelength of the light.

FIG. 16 shows a fiber optic coupler of type <<1 input E towards koutputs, S1, S2, . . . , Sk>>. This component is used in the previousart systems as a type <<1 input towards 2 outputs>>, and is used in thepresent invention as a type <<1 input towards ≧2 outputs . In a fiberoptic coupler, the propagation of light is permitted in both directions,i.e. from the input E to the k outputs S1, S2, . . . , Sk or from eachof the k outputs S1, S2, . . . , Sk to the input E. Thus a light withintensity I at the input E of the coupler is transmitted into the koutputs S1, S2, . . . , Sk, the optical intensity I1, I2, . . . , Ik ofthe transmitted lights into each of the k output fibers S1, S2, . . . ,Sk respectively is equal to I/k. This component operates non selectivelyregarding the wavelength of the light.

FIG. 17 a represents the reflection and transmission properties of a nonwavelength selective mirror 54 lit up by a broad band light source 100.This component is used in the previous art systems and in the presentinvention.

FIG. 17 b represents the reflection and transmission properties of aFiber Bragg Grating 54j_(b,i) lit up by a broad band light source 100:the light reflected by the component is a thin wavelength band aroundthe Bragg wavelength λ_(ji), whereas the transmitted light spectrum isthe complement of the reflected light spectrum.

FIGS. 18 a, 18 b and 18 c represents several reflection and transmissionproperties allowed for mirrors 54 _(ja) (0≦j≦m) of the presentinvention. They can be non wavelength selective (FIG. 18 a), selectivein wavelength bands (FIG. 18 b) or selective in wavelength (FIG. 18 c).

Lit up by a broad band source, the component 54 _(ja) in FIG. 18 bselectively reflects the wavelength band B_(j) and transmits thecomplement spectrum.

Lit up by a broad band source, the component 54 _(ja) in FIG. 18 cselectively reflects the n wavelengths λ_(j1), λ_(j2), . . . , λ_(jn) inthe band B_(j) and transmits the complement spectrum.

FIG. 19 shows a wavelength demultiplexer of type <<1 input towards noutputs>>. This component is used in the previous art systems and in thepresent invention. A wavelength demultiplexer separates the nwavelengths λ₁, λ₂, . . . , λ_(i), . . . , λ_(n) of a light coupled atits input D₀ onto its n outputs D₁, D₂, . . . , D_(j), . . . , D_(n).The light propagation is permitted in the two directions; therefore thecomponent can operate as a wavelength demultiplexer or as a wavelength.

FIG. 20 shows a ‘wavelength dropper’. This component is used in theprevious art systems. A ‘λ_(i) wavelength dropping component’ extractsthe wavelength λ_(i) on its output Ej₂ from an n wavelength λ₁, λ₂, . .. , λ_(i), . . . , λ_(n) light coupled at its input Ej₁; the (n−1)remaining wavelengths λ₁, λ₂, . . . , λ_(i−1), λ_(i+1), . . . , λ_(n)are transmitted on its output E_(j3). The light propagation is permittedin the two directions, therefore the component can operate as a‘wavelength dropper’ or a ‘wavelength insertion’ component.

FIG. 21 shows a wavelength band demultiplexer of type <<1 input towardsm outputs>>. The component is used in the present invention. Awavelength band demultiplexer separates the m wavelength bands B₁, B₂, .. . , B_(j), . . . , B_(n) of a light coupled at its input D₀, into moutputs D₁, D₂, . . . , D_(j), . . . , D_(m). The light propagation ispermitted in the two directions, therefore the component can operate asa wavelength band demultiplexer or multiplexer.

FIG. 22 shows a ‘wavelength band dropper’. This component is used in thepresent invention. A ‘B_(j) wavelength band dropper’ extracts thewavelength band B_(j) on its output Ej₂ from an m wavelength bands B₁,B₂, . . . , B_(j), . . . , B_(m) light coupled at its input Ej₁ ; the(m−1) remaining bands B₁, B₂, . . . , B_(j−1), B_(j+1), . . . , B_(m)are transmitted on its output E_(j3). The light propagation is permittedin the two directions, therefore the component can operate as a‘wavelength band dropper’ or a ‘wavelength band insertion’ component.

FIG. 23 shows an <<inter leaver>> component. The component is used inthe present invention. A light made up of a wavelength comb (λ₁, λ₂, λ₃,λ₄, λ₅, λ₆, . . . , λ_(2n), λ_(2n+1)) with a Δλ wavelength spacing,coupled at the input I of the inter leaver is separated onto its twooutputs S₁ and S₂, into two lights made up of the combs respectively(λ₁, λ₃, λ₅, . . . , λ_(2n+1)) and (λ₂, λ₄, λ₆, . . . , λ_(2n)) with a2.Δλ wavelength spacing. The light propagation is permitted in the twodirections.

The present invention is not limited to the only modes of realizationdescribed above. More precisely, the fiber optic interferometric sensorscan be Fabry Perot interferometers or any interferometric means,selective in wavelength and allowing the proposed time multiplexing.

In the present description, one single reference mirror 54ja (0≦j≦m) isused in per node of sensors, nevertheless, one reference mirror can beused per sensor.

1. Apparatus for multiplexing Fiber Optic Interferometric Sensors(FOISs), which apparatus comprises: means (1) forming an optical sourcefor providing m*n distinct wavelengths λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁,λ₂₂, . . . , λ_(2n), . . . , λ_(m1), λ_(m2), . . . , λ_(mn), the opticalsource including means for generating m groups (111, 112, . . . , 11m)of light pulses, each group 11j (1≦j≦m) being made up of n light pulsesP_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) of distinctwavelengths λ_(j1), λ_(j2), . . . , λ_(jn), the n light pulsesP_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) being time delayedone from another, Fiber Optic Interferometric Sensors arranged toreceive light pulses from the optical source, the sensors distributed inm groups (51, 52, . . . , 5m) of n sensors each, the n sensors of a samegroup being located at the same point and set into a star likearrangement around the input point of the group, and each of the nsensors of a same group 5j, 1≦j≦m, being associated with wavelengthselective means so that it is interrogated by one light pulse only amongthe pulses P_(k)(λ_(j1)), P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) ofdistinct wavelengths respectively λ_(j1), λ_(j2), . . . , λ_(jn). 2.Apparatus according to claim 1, further comprising a referenceinterferometer (2), arranged to receive light pulses comprising anoptical field from the optical source, the reference interferometer madeup of a short arm (2 a) and a long arm (2 b) forming two optical pathsdefining a difference equal to an optical path D, one of the two armscomprising means (21) for actively phase modulating the optical fieldthat propagates through it and arranged to send the modulated opticalfield to the Fiber Optic Interferometric Sensors.
 3. Apparatus accordingto claim 1, in which the means (1) forming the optical source comprisem*n optical sources (611, 612, . . . , 61n, . . . , 621, 622, . . . ,62n, . . . , 6m1, 6m2, . . . , 6mn), a wavelength multiplexer (11), anoptical switch (120) for generating a light pulse P_(k)(λ₁₁, λ₁₂, . . ., λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(m1), λ_(m2), . . . ,λ_(mn)), means (14) for generating, from a light pulse P_(k)(λ₁₁, λ₁₂, .. . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(m1), λ_(m2), . . . ,λ_(mn)), m groups of light pulses 111, 112, . . . , 11m, each of the mgroup 11j, 1≦j≦m, being made up of n light pulses P_(k)(λ_(j1)),P_(k)(λ_(j2)), . . . , P_(k)(λ_(jn)) of distinct wavelengthsrespectively λ_(j1), λ_(j2), . . . , λ_(jn) and delayed one from anotherby a time t(D″), the time t(D″) corresponding to the time of lightpropagation through an optical path D″, within the means (14) forgenerating light pulses, and the time t(D″) being very close to the timet(D) corresponding to the time of light propagation through the opticalpath D.
 4. Apparatus according to claim 3, in which the means (14)comprise an optical fiber (14 ₂) along which n groups G₁, G₂, . . . ,G_(n) of mirrors are successively written, each group G_(i), 1≦i≦n,comprising m mirrors (141 ₁, 142 ₁, . . . , 14mi), the n mirrors (14j₁,14j₂, . . . , 14j_(n)), 1≦j≦m, being spaced one from another on thefiber (14 ₂) by a round trip optical path equal to D″, the m*n mirrors(141 ₁, 142 ₁, . . . , 14m₁, 141 ₂, 142 ₂, . . . , 14m₂, . . . , 141_(n), 142 _(n), . . . , 14m_(n)) selectively reflecting the specificwavelength respectively λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . ,λ_(m1), λ_(m2), . . . , λ_(mn).
 5. Apparatus according to claim 4, inwhich the m mirrors (141 ₁, 142 ₁, . . . , 14m₁, 141 ₂, 142 ₂, . . . ,14m₂, . . . , 141 _(n), 142 _(n), . . . , 14m_(n)) are Fibre BraggGratings.
 6. Apparatus according to claim 3, in which the means (14)comprise a wavelength band demultiplexer (14 ₃) of type 1 input towardsm outputs for separating the m*n specific wavelengths (λ₁₁, λ₁₂, . . . ,λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(m1), λ_(m2), . . . ,λ_(mn)) into m bands B₁=(λ₁₁, λ₁₂, . . . , λ_(1n)), B₂=(λ₂₁, λ₂₂, . . ., λ_(2n)), . . . , B_(m)=(λ_(m1), λ_(m2), . . . , λ_(mn)) of nwavelengths each, towards m means (161, 162, . . . , 16m), each group16j, 1≦j≦m, comprising a wavelength demultiplexer (14j) of type 1 inputtowards n outputs for separating the n wavelengths (λ_(j1), λ_(j2), . .. , λ_(jn)) at its input towards n outputs leading to n mirrors 14j₁,14j₂, . . . , 14j_(n), the n mirrors selectively reflecting thewavelength respectively λ_(j1), λ_(j2), . . . , λ_(jn), and thedifference between the two optical paths delimited, by the mirror 14j₁and the associated output of the demultiplexer (14j), and the mirror14j_(i+1) and the associated output of the demultiplexer (14j), 1≦i≦n,corresponding to the optical path D″/2.
 7. Apparatus according to claim6, in which the m means (16j, 1≦j≦m) each comprise n optical fibers (142_(j1), 142 _(j2), . . . , 142 _(jn)) leading the n wavelengths λ_(j1),λ_(j2), . . . , λ_(jn) out from the wavelength demultiplexer (14j)towards the n mirrors (14j₁, 14j₂, . . . , 14j_(n)).
 8. Apparatusaccording to claim 6, in which mirrors (14j₁, 14j₂, . . . , 14j_(n)),1≦j≦m, are Fiber Bragg Gratings.
 9. Apparatus according to claim 3, inwhich the means (14) comprise a wavelength demultiplexer of type 1 inputtowards m*n outputs for separating the m*n specific wavelengths λ₁₁,λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . ., λ_(2n), . . . , λ_(m1), λ_(m2), .. . , λ_(mn) at its input onto its m*n outputs leading to the m*nmirrors respectively 141 ₁, 141 ₂, . . . , 141 _(n), 142 ₁, 142 ₂, . . ., 142 _(n), . . . , 14m₁, 14m₂, . . . , 14m_(n), the m*n mirrorsselectively reflecting the specific wavelength respectively λ₁₁, λ₁₂, .. . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(m1), λ_(m2), . . . ,λ_(mn), and the difference between the two optical paths delimited, bythe mirror 14j_(i) and the associated output of the demultiplexer, andthe mirror 14j_(i+1) and the associated output of the demultiplexer,1≦i≦n and 1≦j≦m, corresponding to the optical path D″/2.
 10. Apparatusaccording to claim 9, in which the means (14) comprise m*n opticalfibers, leading the m*n wavelengths λ₁₁, λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂,. . . , λ_(2n), . . . , λ_(m1), λ_(m2), . . . , λ_(mn), out of thewavelength demultiplexer of type 1 input towards m*n outputs, to m*nmirrors respectively 141 ₁, 141 ₂, . . . , 141 _(n), 142 ₁, 142 ₂, . . ., 142 _(n), . . . , 14m₁, 14m₂, . . . , 14m_(n).
 11. Apparatus accordingto claim 9, in which the m*n mirrors 141 ₁, 141 ₂, . . . , 141 _(n), 142₁, 142 ₂, . . . , 142 _(n), . . . , 14m₁, 14m₂, . . . , 14m_(n) areFiber Bragg Gratings.
 12. Apparatus according to claim 3, in which themeans (14) comprise means (14 a) for separating the m*n wavelengths λ₁₁,λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . , λ_(m1), λm₂, . .. , λ_(mn) at its input into n packets of m wavelengths each (λ₁₁, λ₂₁,. . . , λ_(m1)), (λ₁₂, λ₂₂, . . . , λ_(m2)), . . . , (λ_(1n), λ_(2n), .. . , λ_(mn)), onto n fibers, respectively 142 ₁, 142 ₂, . . . , 142_(n), and means (14 b) for combining the n packets on a single fibre,the difference of the two optical paths formed by two fiber opticsections 142 _(i) and 142 _((i+1)), 1≦i≦n−1, corresponding to theoptical path D″.
 13. Apparatus according to claim 12, in which means (14a) and (14 b) are formed by a set of inter leavers.
 14. Apparatusaccording to claim 1, in which the means (1) forming the optical sourcecomprise m*n optical sources each emitting a specific wavelength λ₁₁,λ₁₂, . . . , λ_(1n), λ₂₁, λ₂₂, . . . , λ_(2n), . . . λ_(m1), λ_(m2), . .. , λ_(mn), n multiplexers in wavelength 411, 412, . . . , 41n of type minputs towards one output, each of the n wavelength multiplexers 41i,1≦i≦n, combining the m wavelengths λ_(1i), λ_(2i), . . . , λ_(mi) at itsinputs onto its output, n optical switches 121, 122, . . . , 12n at theoutput of the n multiplexers, respectively 411, 412, . . . , 41n forgenerating n light pulses P_(k)(λ₁₁, λ₂₁, . . . , λ_(m1)), P_(k)(λ₁₂,λ₂₂, . . . , λ_(m2)), . . . , P_(k)(λ_(1n), λ_(2n), . . . , λ_(mn))delayed one from another by a time t(D″), means (11) for combining the nlight pulses P_(k)(λ₁₁, λ₂₁, . . . , λ_(m1)), P_(k)(λ₁₂, λ₂₂, . . . ,λ_(m2)), . . . , P_(k)(λ_(1n), λ_(2n), . . . , λ_(mn)) out of the noptical switches 121, 122, . . . , 12n onto a single fiber. 15.Apparatus according to claim 14, which apparatus comprises means fordriving the optical switches.
 16. Apparatus according to claim 1, inwhich the m groups (51, 52, . . . , 5m) of n fiber optic interferometricsensors are distributed along an optical fiber (32) and spaced one fromanother by fiber optic sections (61 _(a), 62 _(a), . . . , 6(m−1)_(a))of any length.
 17. Apparatus according to claim 16, which apparatuscomprises means (61, 62, . . . , 6m) for dropping a part of the lightcarried by optical fiber (32) to the groups respectively 51, 52, . . . ,5m.
 18. Apparatus according to claim 17, in which means (61, 62, . . . ,6m) are fiber optic wavelength band add/drop components associated withthe bands respectively (B₁, B₂, . . . , B_(m)).
 19. Apparatus accordingto claim 1, in which each group 5j, 1≦j≦m, of n fiber opticinterferometric sensors, comprises a fiber optic coupler (51j) of type 1input towards 2 outputs, of which one output is coupled to an opticalfiber (53j_(a)) ended by a reference mirror (54j_(a)), and of which theother output is coupled to a wavelength demultiplexer (52j) of type 1input towards n outputs for separating the n wavelengths of band B_(j)at its input onto n fibers (53j_(b,1), 53j_(b,2), . . . , 53j_(b,n))ended by the mirrors respectively (54j_(b,1), 54j_(b,2), . . . ,54j_(b,n)).
 20. Apparatus according to claim 19, in which for each group5j, 1≦j≦m, of n fiber optic interferometric sensors, the short opticalpath of each of the n sensors corresponds to the optical path betweenthe input of the fiber optic coupler (51j) and the reference mirror(54j_(a)), and the long optical path associated with each of the nsensors corresponds to the optical path between the input of the fiberoptic coupler (51j) and the n mirrors (54j_(b,1), 54j_(b,2), . . . ,54j_(b,n)), the round trip difference between the short and long opticalpaths being equal to D′, and the optical path D′ being very close to theoptical paths D and D″.
 21. Apparatus according to claim 19, in whicheach of the n mirrors (54j_(b,1), 54j_(b,2), . . . , 54j_(b,n)), 1≦j≦m,is a Fiber Bragg Grating selectively reflecting the wavelengthrespectively (λ_(j1), λ_(j2), . . . , λ_(jn)).
 22. Apparatus accordingto claim 19, in which each of the n mirrors (54j_(b,1), 54j_(b,2), . . ., 54j_(b,n)), 1≦j≦m , is not selective in wavelength.
 23. Apparatusaccording to claim 1, in which each group 5j, 1≦j≦m, of n fiber opticinterferometric sensors, comprises a fiber optic coupler of type 1inputtowards n+1 outputs, of which one output is coupled to an optical fiberended by a reference mirror (54j_(a)), and of which the other outputsare coupled in n optical fibers ended by the mirrors respectively54j_(b,1), 54j_(b,2), . . . , 54j_(b,n).
 24. Apparatus according toclaim 23, in which for the group , 1≦j≦m, of n fiber opticinterferometric sensors, the short optical path of each of the n fiberoptic interferometric sensors corresponds to the optical path betweenthe input of the fiber optic coupler and the reference mirror (54j_(a)),and in which the long optical paths associated with each of the nsensors correspond to the optical path between the input of the fiberoptic coupler and the n mirrors (54j_(b,1), 54j_(b,2), . . . ,54j_(b,n)), the round trip difference between the short and long opticalpaths being equal to D′, and the optical path D′ being very close to theoptical paths D and D″.
 25. Apparatus according to claim 23, in whichfor the group 5j, 1≦j≦m, of n fiber optic interferometric sensors, the nmirrors (54j_(b,1), 54j_(b,2), . . . , 54j_(b,n)) are Fiber BraggGratings that selectively reflect the wavelength respectively λ_(j1),λ_(j2), . . . , λ_(jn).
 26. Apparatus according to claim 23, in whichthe reference mirror (54j_(a)) associated with group 5j, 1≦j≦m, is amirror that selectively reflects the wavelength band B_(j). 27.Apparatus according to claim 23, in which the reference mirror (54j_(a))associated with group 5j, 1≦j≦m, is a mirror that selectively reflectsthe wavelengths λ_(j1), λ_(j1), . . . , λ_(jn) of band Bj.
 28. Apparatusaccording to claim 23, in which the reference mirror (54j_(a))associated with group 5j, 1≦j≦m, is a non wavelength selective mirror.29. Apparatus according to claim 2, in which the referenceinterferometer (2) is a Mach-Zehnher interferometer.
 30. Apparatusaccording to claim 1, in which the fiber optic interferometers formingthe sensors are Michelson interferometers.
 31. Apparatus according toclaim 1, which apparatus includes a wavelength band demultiplexer (40),to separate lights of different wavelengths from the optical source, mphoto-detectors (71, 72, . . . 7m) arranged to receive and detectseparated lights from the demultiplexer, and m demodulators (81, 82, . .. , 8m) arranged to demodulate the deteceted lights from thedemultiplexer.
 32. Apparatus according to claim 31, which apparatus iscapable of interrogating one sub array (90), the sub array (90)comprising at least m groups (51, 52, . . . , 5m) of n fiber opticinterferometric sensors, the wavelength band demultiplexer (40), the mphoto-detectors (71, 72, . . . 7m) and the m demodulators (81, 82, . . ., 8m).
 33. Apparatus according to claim 1, which apparatus is capable ofinterrogating I sub arrays (91, 92, . . . , 9 I), each made up of mgroups (51, 52, . . . , 5m) of n sensors, and coupled by means (310) tothe means (1) forming the optical source and the referenceinterferometer (2).
 34. Apparatus according to claim 33, in which themeans (310) are made up of a fiber optic coupler of type 1 input towardsI outputs.